diff --git a/src/ifcviewer-full/MainWindow.cpp b/src/ifcviewer-full/MainWindow.cpp
index 4ac9225985..252278dced 100644
--- a/src/ifcviewer-full/MainWindow.cpp
+++ b/src/ifcviewer-full/MainWindow.cpp
@@ -911,20 +911,32 @@ void MainWindow::onAllLoadsFinished() {
 }
 
 void MainWindow::onObjectPicked(uint32_t object_id) {
-    viewport_->setSelectedObjectId(object_id);
-
+    // Signal originates from the viewport's SelectionState — viewport
+    // selection is already authoritative.  Pushing setSelectedObjectId
+    // back here would clobber a multi-select set with {object_id}.
     auto it = tree_items_.find(object_id);
     if (it != tree_items_.end()) {
         element_tree_->blockSignals(true);
         element_tree_->setCurrentItem(it->second);
         element_tree_->blockSignals(false);
+    } else {
+        // No active (e.g. selection cleared, or active id is in a model
+        // whose tree node hasn't materialised) — drop the tree highlight
+        // so the panel doesn't lie about what's active.
+        element_tree_->blockSignals(true);
+        element_tree_->setCurrentItem(nullptr);
+        element_tree_->blockSignals(false);
     }
 
     populateProperties(object_id);
 
-    if (object_id != 0) {
-        const double v = volumeOfObjects(*viewport_, {object_id});
-        qInfo("Volume of object %u: %.6f m^3", object_id, v);
+    // Volume readout: report for the full selection so multi-select
+    // matches the highlighted set.
+    const auto& selection = viewport_->selection().selectionIds();
+    if (!selection.empty()) {
+        std::vector<uint32_t> ids(selection.begin(), selection.end());
+        const double v = volumeOfObjects(*viewport_, ids);
+        qInfo("Volume of %zu selected object(s): %.6f m^3", ids.size(), v);
     }
 }
 
@@ -933,6 +945,8 @@ void MainWindow::onTreeSelectionChanged() {
     if (items.isEmpty()) return;
 
     uint32_t object_id = items.first()->data(0, Qt::UserRole).toUInt();
+    // Tree stays single-select per UX choice — clicking a tree item
+    // replaces the viewport's multi-set with just that item.
     viewport_->setSelectedObjectId(object_id);
     populateProperties(object_id);
 }
diff --git a/src/ifcviewer/OverlayRenderer.cpp b/src/ifcviewer/OverlayRenderer.cpp
index e477e73717..228b2ee49c 100644
--- a/src/ifcviewer/OverlayRenderer.cpp
+++ b/src/ifcviewer/OverlayRenderer.cpp
@@ -382,6 +382,10 @@ void OverlayRenderer::setOverlayLabels(const std::vector<Label>& labels) {
     labels_ = labels;
 }
 
+void OverlayRenderer::setSelectionRect(const QRect& rect_logical) {
+    selection_rect_ = rect_logical;
+}
+
 void OverlayRenderer::setHighlightTriangles(const std::vector<float>& world_xyz,
                                              float r, float g, float b, float a) {
     if (!gl_) return;
@@ -512,6 +516,90 @@ void OverlayRenderer::render(const float view_proj[16],
     gl_->glDepthMask(prev_depth_msk);
     gl_->glDepthFunc(prev_depth_func);
 
+    if (pixel_w <= 0 || pixel_h <= 0) return;
+
+    const float logical_w = float(pixel_w) / float(dpr ? dpr : 1.0);
+    const float logical_h = float(pixel_h) / float(dpr ? dpr : 1.0);
+    auto px_to_ndc_x = [logical_w](float px) {
+        return (px / logical_w) * 2.0f - 1.0f;
+    };
+    auto px_to_ndc_y = [logical_h](float px) {
+        return 1.0f - (px / logical_h) * 2.0f;
+    };
+
+    // Box-select rectangle: translucent fill + 1-px outline drawn as
+    // four thin rects.  Comes before the HUD/label pass so the HUD
+    // backgrounds still render on top of the rectangle if they overlap.
+    if (selection_rect_.isValid()
+        && selection_rect_.width()  > 0
+        && selection_rect_.height() > 0) {
+        struct RectPx { float x0, y0, x1, y1; };
+        const QRect& sr = selection_rect_;
+        const float sx0 = float(sr.left());
+        const float sy0 = float(sr.top());
+        const float sx1 = float(sr.right() + 1);
+        const float sy1 = float(sr.bottom() + 1);
+        const RectPx pieces[5] = {
+            // Fill
+            {sx0, sy0, sx1, sy1},
+            // Top edge
+            {sx0, sy0, sx1, sy0 + 1.0f},
+            // Bottom edge
+            {sx0, sy1 - 1.0f, sx1, sy1},
+            // Left edge
+            {sx0, sy0, sx0 + 1.0f, sy1},
+            // Right edge
+            {sx1 - 1.0f, sy0, sx1, sy1},
+        };
+        const float colors[5][4] = {
+            {0.30f, 0.65f, 1.0f, 0.18f},  // fill
+            {0.30f, 0.65f, 1.0f, 0.9f},   // outline (each edge)
+            {0.30f, 0.65f, 1.0f, 0.9f},
+            {0.30f, 0.65f, 1.0f, 0.9f},
+            {0.30f, 0.65f, 1.0f, 0.9f},
+        };
+
+        GLboolean prev_dt2  = gl_->glIsEnabled(GL_DEPTH_TEST);
+        GLboolean prev_cf2  = gl_->glIsEnabled(GL_CULL_FACE);
+        GLboolean prev_bl2  = gl_->glIsEnabled(GL_BLEND);
+        gl_->glDisable(GL_DEPTH_TEST);
+        gl_->glDisable(GL_CULL_FACE);
+        gl_->glEnable(GL_BLEND);
+        gl_->glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
+        gl_->glUseProgram(program_rect_);
+        gl_->glBindVertexArray(vao_rect_);
+
+        // Each piece is its own draw so we can switch alpha between
+        // fill and outline.  All five share the same VBO slot — we
+        // stream-overwrite per draw.
+        const size_t bytes = 12 * sizeof(float);
+        if (bytes > vbo_rect_capacity_) {
+            const size_t new_cap = bytes + bytes / 2;
+            gl_->glNamedBufferData(vbo_rect_, GLsizeiptr(new_cap),
+                                   nullptr, GL_DYNAMIC_DRAW);
+            vbo_rect_capacity_ = new_cap;
+        }
+        for (int i = 0; i < 5; ++i) {
+            const float x0 = px_to_ndc_x(pieces[i].x0);
+            const float x1 = px_to_ndc_x(pieces[i].x1);
+            const float y0 = px_to_ndc_y(pieces[i].y0);
+            const float y1 = px_to_ndc_y(pieces[i].y1);
+            const float ndc[12] = {
+                x0, y0,  x1, y0,  x0, y1,
+                x0, y1,  x1, y0,  x1, y1
+            };
+            gl_->glNamedBufferSubData(vbo_rect_, 0, GLsizeiptr(bytes), ndc);
+            gl_->glUniform4f(u_rect_color_,
+                             colors[i][0], colors[i][1],
+                             colors[i][2], colors[i][3]);
+            gl_->glDrawArrays(GL_TRIANGLES, 0, 6);
+        }
+        gl_->glBindVertexArray(0);
+        if (prev_dt2) gl_->glEnable(GL_DEPTH_TEST);
+        if (prev_cf2) gl_->glEnable(GL_CULL_FACE);
+        if (!prev_bl2) gl_->glDisable(GL_BLEND);
+    }
+
     // Two-stage HUD/label pass: collect rect bounds (in logical pixels) +
     // text strings, draw all rect backgrounds via GL (screen-space NDC
     // quads, depth test off), then run a QPainter pass that *only* draws
@@ -519,10 +607,7 @@ void OverlayRenderer::render(const float view_proj[16],
     // QOpenGLPaintDevice quirk where solid fills silently drop while
     // text continues to render.
     const bool any_painter = !hud_text_.isEmpty() || !labels_.empty();
-    if (!any_painter || pixel_w <= 0 || pixel_h <= 0) return;
-
-    const float logical_w = float(pixel_w) / float(dpr ? dpr : 1.0);
-    const float logical_h = float(pixel_h) / float(dpr ? dpr : 1.0);
+    if (!any_painter) return;
 
     QFont label_font("monospace", 9);
     label_font.setStyleHint(QFont::TypeWriter);
@@ -577,12 +662,6 @@ void OverlayRenderer::render(const float view_proj[16],
     if (!items.empty()) {
         std::vector<float> ndc;
         ndc.reserve(items.size() * 12);   // 6 verts * 2 floats per rect
-        auto px_to_ndc_x = [logical_w](float px) {
-            return (px / logical_w) * 2.0f - 1.0f;
-        };
-        auto px_to_ndc_y = [logical_h](float px) {
-            return 1.0f - (px / logical_h) * 2.0f;
-        };
         for (const auto& it : items) {
             const float x0 = px_to_ndc_x(float(it.bg.left()));
             const float x1 = px_to_ndc_x(float(it.bg.right() + 1));
diff --git a/src/ifcviewer/OverlayRenderer.h b/src/ifcviewer/OverlayRenderer.h
index d3a0665839..3738d4755b 100644
--- a/src/ifcviewer/OverlayRenderer.h
+++ b/src/ifcviewer/OverlayRenderer.h
@@ -20,6 +20,7 @@
 #ifndef IFCVIEWER_OVERLAYRENDERER_H
 #define IFCVIEWER_OVERLAYRENDERER_H
 
+#include <QRect>
 #include <QString>
 #include <QtOpenGL/QOpenGLFunctions_4_5_Core>
 
@@ -86,6 +87,12 @@ public:
     // render().  Empty hides the HUD.
     void setHudText(const QString& text);
 
+    // Box-select rectangle (in logical pixel coords, top-left origin).
+    // Drawn as a translucent fill + 1-px outline using the same
+    // screen-space rect program that draws label/HUD backgrounds.
+    // Empty rect hides it.
+    void setSelectionRect(const QRect& rect_logical);
+
     // Render every overlay primitive in order: GL highlight triangles
     // (using `view_proj`, column-major float[16]), then HUD text via
     // QPainter on a QOpenGLPaintDevice sized to (pixel_w × pixel_h)
@@ -182,6 +189,9 @@ private:
 
     std::vector<Label> labels_;
     QString            hud_text_;
+
+    // Box-select rectangle in logical pixels.  Null/empty = hidden.
+    QRect              selection_rect_;
 };
 
 #endif // IFCVIEWER_OVERLAYRENDERER_H
diff --git a/src/ifcviewer/Selection.cpp b/src/ifcviewer/Selection.cpp
new file mode 100644
index 0000000000..4ecde55381
--- /dev/null
+++ b/src/ifcviewer/Selection.cpp
@@ -0,0 +1,184 @@
+/********************************************************************************
+ *                                                                              *
+ * This file is part of IfcOpenShell.                                           *
+ *                                                                              *
+ * IfcOpenShell is free software: you can redistribute it and/or modify         *
+ * it under the terms of the Lesser GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3.0 of the License, or          *
+ * (at your option) any later version.                                          *
+ *                                                                              *
+ * IfcOpenShell is distributed in the hope that it will be useful,              *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
+ * Lesser GNU General Public License for more details.                          *
+ *                                                                              *
+ * You should have received a copy of the Lesser GNU General Public License     *
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
+ *                                                                              *
+ ********************************************************************************/
+
+#include "Selection.h"
+
+#include <algorithm>
+
+SelectionState::SelectionState(QObject* parent) : QObject(parent) {
+    // Reserve slot 0 — object_id 0 is the "no object" sentinel and the
+    // shader still indexes the buffer with v_object_id, so the slot must
+    // exist (and be zero) to avoid OOB reads.
+    cpu_flags_.assign(1, 0u);
+}
+
+SelectionState::~SelectionState() = default;
+
+void SelectionState::initializeGl(QOpenGLFunctions_4_5_Core* gl) {
+    gl_ = gl;
+    if (ssbo_ == 0) {
+        gl_->glCreateBuffers(1, &ssbo_);
+    }
+    // Force a fresh upload against the new context.
+    dirty_ = true;
+}
+
+void SelectionState::releaseGl() {
+    if (gl_ && ssbo_) {
+        gl_->glDeleteBuffers(1, &ssbo_);
+    }
+    ssbo_ = 0;
+    ssbo_capacity_slots_ = 0;
+    gl_ = nullptr;
+    dirty_ = true;
+}
+
+void SelectionState::reset() {
+    const bool had_state = !selected_ids_.empty() || active_id_ != 0;
+    selected_ids_.clear();
+    active_id_ = 0;
+    std::fill(cpu_flags_.begin(), cpu_flags_.end(), 0u);
+    markDirty();
+    if (had_state) emit changed(active_id_);
+}
+
+void SelectionState::noteObjectId(uint32_t id) {
+    if (id == 0) return;
+    if (uint32_t(cpu_flags_.size()) <= id) {
+        // Grow CPU side; the SSBO is sized lazily inside uploadFlags so
+        // we don't realloc GL on every streamed instance.
+        cpu_flags_.resize(size_t(id) + 1, 0u);
+        markDirty();
+    }
+}
+
+void SelectionState::setSelectedObjectId(uint32_t id) {
+    if (id == 0) {
+        clearSelection();
+        return;
+    }
+    setSelection({id}, id);
+}
+
+void SelectionState::setSelection(const std::unordered_set<uint32_t>& ids,
+                                  uint32_t active) {
+    // Avoid emitting churn when the call is a no-op.
+    if (ids == selected_ids_ && active == active_id_) return;
+    selected_ids_ = ids;
+    selected_ids_.erase(0);
+    active_id_ = (active != 0 && selected_ids_.count(active)) ? active : 0u;
+    markDirty();
+    emit changed(active_id_);
+}
+
+void SelectionState::addToSelection(const std::unordered_set<uint32_t>& ids) {
+    bool any_added = false;
+    for (uint32_t id : ids) {
+        if (id == 0) continue;
+        if (selected_ids_.insert(id).second) any_added = true;
+    }
+    if (!any_added) return;
+    markDirty();
+    emit changed(active_id_);
+}
+
+void SelectionState::removeFromSelection(const std::unordered_set<uint32_t>& ids) {
+    bool any_removed = false;
+    bool active_removed = false;
+    for (uint32_t id : ids) {
+        if (selected_ids_.erase(id) > 0) {
+            any_removed = true;
+            if (id == active_id_) active_removed = true;
+        }
+    }
+    if (!any_removed) return;
+    if (active_removed) active_id_ = 0;
+    markDirty();
+    emit changed(active_id_);
+}
+
+void SelectionState::toggleInSelection(uint32_t id) {
+    if (id == 0) return;
+    if (selected_ids_.erase(id) > 0) {
+        // Removed.  If it was active, drop active.
+        if (active_id_ == id) active_id_ = 0;
+    } else {
+        // Added.  Last-toggled becomes active so the properties panel
+        // tracks the most recent click — matches the user's "last single
+        // clicked is active" expectation.
+        selected_ids_.insert(id);
+        active_id_ = id;
+    }
+    markDirty();
+    emit changed(active_id_);
+}
+
+void SelectionState::clearSelection() {
+    if (selected_ids_.empty() && active_id_ == 0) return;
+    selected_ids_.clear();
+    active_id_ = 0;
+    markDirty();
+    emit changed(active_id_);
+}
+
+void SelectionState::markDirty() {
+    dirty_ = true;
+}
+
+void SelectionState::growTo(uint32_t capacity_ids) {
+    if (!gl_ || ssbo_ == 0) return;
+    if (capacity_ids <= ssbo_capacity_slots_) return;
+    // Round up to a power-of-two-ish step so streamed scenes don't realloc
+    // every few instances.  Floor at 1024 slots = 4 KB.
+    size_t new_cap = std::max<size_t>(1024, ssbo_capacity_slots_ * 2);
+    while (new_cap < capacity_ids) new_cap *= 2;
+    gl_->glNamedBufferData(ssbo_,
+                           GLsizeiptr(new_cap * sizeof(uint32_t)),
+                           nullptr, GL_DYNAMIC_DRAW);
+    ssbo_capacity_slots_ = new_cap;
+}
+
+void SelectionState::uploadFlags() {
+    if (!gl_ || ssbo_ == 0) return;
+
+    // Rebuild the CPU flag vector from the canonical selected_ids_.  The
+    // overhead is O(|cpu_flags_|), which scales with max object_id rather
+    // than with selection size — acceptable: object_ids are dense so this
+    // is just a memset + a handful of writes for the selected set.
+    std::fill(cpu_flags_.begin(), cpu_flags_.end(), 0u);
+    for (uint32_t id : selected_ids_) {
+        if (id < cpu_flags_.size()) cpu_flags_[id] = 1u;
+    }
+
+    growTo(static_cast<uint32_t>(cpu_flags_.size()));
+    if (ssbo_capacity_slots_ == 0) return;
+
+    const GLsizeiptr bytes =
+        GLsizeiptr(cpu_flags_.size() * sizeof(uint32_t));
+    if (bytes > 0) {
+        gl_->glNamedBufferSubData(ssbo_, 0, bytes, cpu_flags_.data());
+    }
+    dirty_ = false;
+}
+
+void SelectionState::bindForRender(GLuint binding_index) {
+    if (!gl_ || ssbo_ == 0) return;
+    if (dirty_) uploadFlags();
+    gl_->glBindBufferBase(GL_SHADER_STORAGE_BUFFER, binding_index, ssbo_);
+}
diff --git a/src/ifcviewer/Selection.h b/src/ifcviewer/Selection.h
new file mode 100644
index 0000000000..7d0e84f98f
--- /dev/null
+++ b/src/ifcviewer/Selection.h
@@ -0,0 +1,126 @@
+/********************************************************************************
+ *                                                                              *
+ * This file is part of IfcOpenShell.                                           *
+ *                                                                              *
+ * IfcOpenShell is free software: you can redistribute it and/or modify         *
+ * it under the terms of the Lesser GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3.0 of the License, or          *
+ * (at your option) any later version.                                          *
+ *                                                                              *
+ * IfcOpenShell is distributed in the hope that it will be useful,              *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
+ * Lesser GNU General Public License for more details.                          *
+ *                                                                              *
+ * You should have received a copy of the Lesser GNU General Public License     *
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
+ *                                                                              *
+ ********************************************************************************/
+
+#ifndef IFCVIEWER_SELECTION_H
+#define IFCVIEWER_SELECTION_H
+
+#include <QObject>
+#include <QtOpenGL/QOpenGLFunctions_4_5_Core>
+
+#include <cstdint>
+#include <unordered_set>
+#include <vector>
+
+// Multi-selection state for the viewport, owned by ViewportWindow.
+//
+// Tracks the *set* of currently selected object_ids plus a single "active"
+// id — the last single-clicked one.  The active id is what the properties
+// panel and tree mirror; the full set is what the viewport highlights.
+//
+// Selection state is published to the main shader through a per-object_id
+// flags SSBO bound at a caller-chosen index (binding=3 today).  The buffer
+// is sized to max(object_id) + 1 and grown on demand via noteObjectId,
+// which the viewport calls for every appended instance.
+//
+// On every mutation the selection emits `changed(active_id)`.  Consumers
+// (MainWindow, the viewport itself for requestUpdate) connect to it.
+class SelectionState : public QObject {
+    Q_OBJECT
+public:
+    explicit SelectionState(QObject* parent = nullptr);
+    ~SelectionState() override;
+
+    // Wire up the GL context.  Must be called once the viewport's GL
+    // context is current.  release() drops GL resources before context
+    // teardown.
+    void initializeGl(QOpenGLFunctions_4_5_Core* gl);
+    void releaseGl();
+
+    // Tell the manager about a newly added object_id so the flag buffer
+    // can grow ahead of the next render.  Cheap when the id fits in the
+    // already-allocated CPU vector; otherwise resizes (and marks dirty).
+    void noteObjectId(uint32_t id);
+
+    // Clear everything — both the selection set and the per-object flags.
+    // Called from clearScene.
+    void reset();
+
+    // ---- Mutation API ----
+    //
+    // Plain LMB click → setSelectedObjectId(id) (or clearSelection() for 0).
+    // Modifier+click  → toggleInSelection(id).
+    // Box-select on release → setSelection / addToSelection / removeFromSelection
+    // depending on the modifier held when the drag started.
+    //
+    // setSelection's `active` should be in `ids` or 0; if it isn't, the
+    // active is silently coerced to 0.
+
+    void setSelectedObjectId(uint32_t id);
+    void setSelection(const std::unordered_set<uint32_t>& ids, uint32_t active);
+    void addToSelection(const std::unordered_set<uint32_t>& ids);
+    void removeFromSelection(const std::unordered_set<uint32_t>& ids);
+    void toggleInSelection(uint32_t id);
+    void clearSelection();
+
+    // ---- Accessors ----
+
+    bool isSelected(uint32_t id) const { return selected_ids_.count(id) > 0; }
+    bool empty() const { return selected_ids_.empty(); }
+    size_t size() const { return selected_ids_.size(); }
+    const std::unordered_set<uint32_t>& selectionIds() const { return selected_ids_; }
+    uint32_t activeObjectId() const { return active_id_; }
+
+    // ---- GL binding ----
+    //
+    // Bind the selection-flags SSBO at the given binding index for the
+    // upcoming draw.  Lazily uploads any pending flag changes.  Caller
+    // must have GL context current.
+    void bindForRender(GLuint binding_index);
+
+signals:
+    // Emitted on any mutation that changes either the set or the active.
+    // Carries the new active id for convenience (consumers usually only
+    // care about the active for properties/tree sync).
+    void changed(uint32_t active_id);
+
+private:
+    // Mark the SSBO dirty so the next bindForRender() re-uploads it.
+    void markDirty();
+    // Resize the CPU flag vector + GL buffer to hold up to capacity_ids
+    // entries.  Called when noteObjectId outgrows the current capacity.
+    void growTo(uint32_t capacity_ids);
+    // Fully overwrite cpu_flags_ from selected_ids_, then upload to GL.
+    void uploadFlags();
+
+    QOpenGLFunctions_4_5_Core* gl_ = nullptr;
+
+    std::unordered_set<uint32_t> selected_ids_;
+    uint32_t active_id_ = 0;
+
+    // Per-object_id flag, indexed by id directly (slot 0 unused — object_id 0
+    // means "no object").  Bit 0 = selected.  Stored as uint32 per slot for
+    // std430 alignment simplicity; the byte cost (~4 MB at 1M objects) is
+    // negligible compared to the instance SSBO.
+    std::vector<uint32_t> cpu_flags_;
+    GLuint ssbo_                = 0;
+    size_t ssbo_capacity_slots_ = 0;
+    bool   dirty_               = true;
+};
+
+#endif // IFCVIEWER_SELECTION_H
diff --git a/src/ifcviewer/ViewportWindow.cpp b/src/ifcviewer/ViewportWindow.cpp
index b01bec58c7..ca725ea3b6 100644
--- a/src/ifcviewer/ViewportWindow.cpp
+++ b/src/ifcviewer/ViewportWindow.cpp
@@ -88,15 +88,21 @@ struct MeshQuant { vec4 aabb_min; vec4 aabb_max; };
 layout(std430, binding = 2) readonly buffer Meshes {
     MeshQuant meshes[];
 };
+// Per-object_id selection flag (bit 0 = in selection).  Sized so that
+// any object_id present in the scene is a valid index — see SelectionState.
+layout(std430, binding = 3) readonly buffer SelectionFlags {
+    uint sel_flags[];
+};
 
 uniform mat4 u_view_projection;
-uniform uint u_selected_id;
+uniform uint u_active_id;       // last single-clicked id (0 = none)
 
 out vec3 v_normal;
 out vec4 v_color;
 out vec3 v_world_pos;
 flat out uint v_object_id;
 flat out uint v_selected;
+flat out uint v_active;
 
 // Meyer et al. octahedral normal decode.  Input is in [-1,1]^2.
 vec3 octDecode(vec2 e) {
@@ -143,7 +149,10 @@ void main() {
     v_color = baked;
 
     v_object_id = inst.object_id;
-    v_selected = (v_object_id == u_selected_id) ? 1u : 0u;
+    // Index by object_id directly; SelectionState guarantees the buffer
+    // is sized to cover every id allocated to the scene.
+    v_selected  = ((sel_flags[v_object_id] & 1u) != 0u) ? 1u : 0u;
+    v_active    = (v_object_id == u_active_id) ? 1u : 0u;
 }
 )";
 
@@ -154,6 +163,7 @@ in vec4 v_color;
 in vec3 v_world_pos;
 flat in uint v_object_id;
 flat in uint v_selected;
+flat in uint v_active;
 
 uniform vec3 u_light_dir;       // primary key direction (world-space)
 uniform vec3 u_fill_dir;        // secondary fill direction
@@ -202,7 +212,11 @@ void main() {
     float cavity = clamp(length(fwidth(n)) * 1.5, 0.0, 0.35);
     color *= (1.0 - cavity);
 
-    if (v_selected == 1u) color = mix(color, vec3(0.2, 0.6, 1.0), 0.5);
+    // Selected (member of the multi-set) gets a blue tint; the active
+    // (last single-clicked) gets a stronger mix so the user can always
+    // tell which one drives the properties panel.
+    if (v_selected == 1u) color = mix(color, vec3(0.2, 0.6, 1.0), 0.45);
+    if (v_active   == 1u) color = mix(color, vec3(0.4, 0.8, 1.0), 0.40);
     frag_color = vec4(color, v_color.a);
 }
 )";
@@ -671,6 +685,7 @@ ViewportWindow::~ViewportWindow() {
             if (hiz_downsample_program_) gl_->glDeleteProgram(hiz_downsample_program_);
             if (hiz_downsample_vao_) gl_->glDeleteVertexArrays(1, &hiz_downsample_vao_);
             overlay_renderer_.release();
+            selection_.releaseGl();
         }
         context_->doneCurrent();
     }
@@ -692,6 +707,13 @@ void ViewportWindow::initGL() {
     buildPivotIndicator();
     buildSectionPlaneGizmo();
     overlay_renderer_.initialize(gl_);
+    selection_.initializeGl(gl_);
+    // Drive viewport repaints + tree/properties sync from selection state.
+    connect(&selection_, &SelectionState::changed, this,
+            [this](uint32_t active_id) {
+                requestUpdate();
+                emit objectPicked(active_id);
+            });
 
     gl_->glEnable(GL_DEPTH_TEST);
     gl_->glEnable(GL_MULTISAMPLE);
@@ -1074,6 +1096,8 @@ void ViewportWindow::uploadInstanceChunk(const InstanceChunk& chunk) {
 
     ModelGpuData& m = getOrCreateModel(chunk.model_id);
 
+    selection_.noteObjectId(chunk.object_id);
+
     InstanceCpu inst;
     inst.mesh_id  = chunk.local_mesh_id;
     inst.object_id = chunk.object_id;
@@ -1235,6 +1259,14 @@ void ViewportWindow::applyCachedModel(uint32_t model_id, SidecarData data) {
     m.meshes = std::move(data.meshes);
     m.instances = std::move(data.instances);
 
+    // Sidecar path bypasses uploadInstanceChunk, so register every
+    // instance's object_id with the selection state up front — otherwise
+    // the per-object_id flags SSBO would be too small for these ids and
+    // their selection bit reads would silently fall outside the buffer.
+    for (const auto& inst : m.instances) {
+        selection_.noteObjectId(inst.object_id);
+    }
+
     uint32_t total_tri = 0;
     for (const auto& mesh : m.meshes) {
         total_tri += (mesh.index_count / 3) * mesh.instance_count;
@@ -1374,7 +1406,7 @@ void ViewportWindow::resetScene() {
         if (m.indirect_buffer) gl_->glDeleteBuffers(1, &m.indirect_buffer);
     }
     models_gpu_.clear();
-    selected_object_id_ = 0;
+    selection_.reset();
     have_cached_cull_ = false;
     requestUpdate();
 }
@@ -1436,8 +1468,10 @@ void ViewportWindow::removeModel(uint32_t model_id) {
 }
 
 void ViewportWindow::setSelectedObjectId(uint32_t id) {
-    selected_object_id_ = id;
-    requestUpdate();
+    // Convenience pass-through.  SelectionState fires `changed` which
+    // re-emits objectPicked + requestUpdate via the lambda hooked up in
+    // initGL, so we don't need to do those manually here.
+    selection_.setSelectedObjectId(id);
 }
 
 void ViewportWindow::setCamera(float tx, float ty, float tz,
@@ -1534,8 +1568,12 @@ void ViewportWindow::frameAabb(const QVector3D& mn, const QVector3D& mx,
 
 void ViewportWindow::focusOnSelectedObject() {
     if (camera_mode_ == CameraMode::Fps) return;
+    // F frames the *active* object so it's predictable across multi-select
+    // states.  Framing the union of every selected member would be more
+    // expansive but ambiguous in a large set.
+    const uint32_t target = selection_.activeObjectId();
     QVector3D mn, mx;
-    if (!computeObjectAabb(selected_object_id_, mn, mx)) {
+    if (!computeObjectAabb(target, mn, mx)) {
         qDebug("Focus: no object selected or no AABB available");
         return;
     }
@@ -2079,6 +2117,59 @@ uint32_t ViewportWindow::pickObjectAt(int x, int y) {
     return pixel;
 }
 
+std::unordered_set<uint32_t> ViewportWindow::picksInRect(const QRect& rect_logical) {
+    std::unordered_set<uint32_t> out;
+    if (!gl_initialized_) return out;
+    if (rect_logical.isEmpty() || rect_logical.width() < 1 || rect_logical.height() < 1) {
+        return out;
+    }
+    context_->makeCurrent(this);
+
+    const int w = width()  * devicePixelRatio();
+    const int h = height() * devicePixelRatio();
+    if (w <= 0 || h <= 0) return out;
+
+    // Reuse the same allocation path as pickObjectAt — call it once at
+    // (0,0) just to ensure the framebuffer matches the current size and
+    // the pick pass has been rendered for this state.
+    pickObjectAt(rect_logical.left(), rect_logical.top());
+
+    // Convert the logical-coords rect to physical pick-tex coords with
+    // GL's bottom-left origin.  Clamp to surface to avoid OOB reads.
+    const qreal dpr = devicePixelRatio();
+    QRect r = rect_logical
+        .intersected(QRect(0, 0, width(), height()));
+    if (r.isEmpty()) return out;
+    const int rx = int(r.left() * dpr);
+    const int ry_top = int((height() - (r.top() + r.height())) * dpr);
+    const int rw = std::max(1, int(r.width()  * dpr));
+    const int rh = std::max(1, int(r.height() * dpr));
+    if (rx < 0 || ry_top < 0 || rx + rw > w || ry_top + rh > h) {
+        // Final defensive clamp — devicePixelRatio + integer rounding can
+        // push the rect a pixel off the texture; readback would silently
+        // return zeros for those rows.
+        const int cx  = std::max(0, std::min(rx, w - 1));
+        const int cy  = std::max(0, std::min(ry_top, h - 1));
+        const int cw2 = std::max(1, std::min(rw, w - cx));
+        const int ch2 = std::max(1, std::min(rh, h - cy));
+        std::vector<uint32_t> pixels(size_t(cw2) * size_t(ch2), 0);
+        gl_->glGetTextureSubImage(pick_color_tex_, 0, cx, cy, 0, cw2, ch2, 1,
+                                  GL_RED_INTEGER, GL_UNSIGNED_INT,
+                                  GLsizei(pixels.size() * sizeof(uint32_t)),
+                                  pixels.data());
+        for (uint32_t id : pixels) if (id != 0) out.insert(id);
+        return out;
+    }
+
+    std::vector<uint32_t> pixels(size_t(rw) * size_t(rh), 0);
+    gl_->glGetTextureSubImage(pick_color_tex_, 0, rx, ry_top, 0, rw, rh, 1,
+                              GL_RED_INTEGER, GL_UNSIGNED_INT,
+                              GLsizei(pixels.size() * sizeof(uint32_t)),
+                              pixels.data());
+    for (uint32_t id : pixels) if (id != 0) out.insert(id);
+    return out;
+}
+
 bool ViewportWindow::pickSurfaceAt(int x, int y,
                                    uint32_t& object_id_out,
                                    QVector3D& world_pos_out,
@@ -2565,7 +2656,7 @@ void ViewportWindow::render() {
     GLint u_fill      = gl_->glGetUniformLocation(main_program_, "u_fill_dir");
     GLint u_sky       = gl_->glGetUniformLocation(main_program_, "u_sky_color");
     GLint u_ground    = gl_->glGetUniformLocation(main_program_, "u_ground_color");
-    GLint u_sel       = gl_->glGetUniformLocation(main_program_, "u_selected_id");
+    GLint u_active    = gl_->glGetUniformLocation(main_program_, "u_active_id");
     gl_->glUniformMatrix4fv(u_vp, 1, GL_FALSE, vp.constData());
     // Key light: high noon-ish from off-camera; fill ~120° away so back-of-
     // object surfaces still get some direct contribution.  Sky/ground tints
@@ -2577,7 +2668,8 @@ void ViewportWindow::render() {
     gl_->glUniform3f(u_fill,  -0.3f, -0.5f, 0.8f);
     gl_->glUniform3f(u_sky,    0.55f, 0.60f, 0.70f);
     gl_->glUniform3f(u_ground, 0.35f, 0.32f, 0.28f);
-    gl_->glUniform1ui(u_sel, selected_object_id_);
+    gl_->glUniform1ui(u_active, selection_.activeObjectId());
+    selection_.bindForRender(/*binding_index=*/3);
     uploadClipPlaneUniforms(main_program_);
 
     visible_triangles_ = 0;
@@ -2740,6 +2832,14 @@ void ViewportWindow::render() {
     // does not restore.
     {
         const qreal dpr = devicePixelRatio();
+        // Push the live box-select rect (or empty when not dragging) so
+        // the overlay draws/clears it as part of the per-frame pass.
+        QRect sel_rect;
+        if (box_select_active_) {
+            sel_rect = QRect(box_select_start_pos_,
+                             box_select_current_pos_).normalized();
+        }
+        overlay_renderer_.setSelectionRect(sel_rect);
         overlay_renderer_.render(vp.constData(),
                                  int(width()  * dpr),
                                  int(height() * dpr),
@@ -3455,6 +3555,9 @@ void ViewportWindow::handleMousePress(QMouseEvent* e) {
     }
     active_button_ = e->button();
     last_mouse_pos_ = e->pos();
+    box_select_armed_       = false;
+    box_select_active_      = false;
+    press_pick_id_          = 0;
     if (e->button() == Qt::MiddleButton) {
         setPivotIndicatorVisible(true);
         requestUpdate();
@@ -3483,6 +3586,23 @@ void ViewportWindow::handleMousePress(QMouseEvent* e) {
             section_plane_selected_ = -1;
             requestUpdate();
         }
+        return;
+    }
+
+    // Plain LMB: pick now so release can apply click semantics without
+    // a second pick pass, and arm a potential box-select drag — promoted
+    // to box_select_active_ at the first move past the click threshold.
+    // Arming on every LMB press (not just empty-space presses) means a
+    // drag that happens to start on an object still box-selects, which
+    // matches user intuition: the start point shouldn't disqualify the
+    // gesture.  Tool-mode LMB defers pick handling to surfacePickedInTool
+    // on release.
+    if (e->button() == Qt::LeftButton && tool_mode_ == ToolMode::None) {
+        press_pick_id_ = pickObjectAt(e->pos().x(), e->pos().y());
+        box_select_start_pos_   = e->pos();
+        box_select_current_pos_ = e->pos();
+        box_select_press_mods_  = e->modifiers();
+        box_select_armed_       = true;
     }
 }
 void ViewportWindow::handleMouseRelease(QMouseEvent* e) {
@@ -3493,22 +3613,57 @@ void ViewportWindow::handleMouseRelease(QMouseEvent* e) {
         active_button_ = Qt::NoButton;
         return;
     }
-    // LMB pick is suppressed in section-tool mode — LMB there creates or
-    // selects planes (handled in handleMousePress) and the release should
-    // not also trigger object selection.
-    if (active_button_ == Qt::LeftButton
-        && !section_tool_active_
-        && (e->pos() - last_mouse_pos_).manhattanLength() < 5) {
-        if (tool_mode_ != ToolMode::None) {
-            emit surfacePickedInTool(e->pos().x(), e->pos().y(),
-                                     int(e->modifiers()));
-        } else {
-            uint32_t id = pickObjectAt(e->pos().x(), e->pos().y());
-            selected_object_id_ = id;
-            emit objectPicked(id);
-            requestUpdate();  // selection highlight changed
+
+    if (active_button_ == Qt::LeftButton && !section_tool_active_) {
+        const bool was_drag =
+            (e->pos() - box_select_start_pos_).manhattanLength() >= 5;
+
+        if (box_select_active_) {
+            // Finalize a box-select.  Modifier captured at press-time
+            // decides commit semantics: plain → replace, Shift → add,
+            // Ctrl → remove.  Box-select never sets the active id; the
+            // last single-clicked object remains the active.
+            const QRect rect = QRect(box_select_start_pos_,
+                                     e->pos()).normalized();
+            const auto picks = picksInRect(rect);
+            const auto mods  = box_select_press_mods_;
+            if (mods & Qt::ShiftModifier) {
+                selection_.addToSelection(picks);
+            } else if (mods & Qt::ControlModifier) {
+                selection_.removeFromSelection(picks);
+            } else {
+                // Replace.  Active stays only if the new set still
+                // contains it; SelectionState handles the coercion.
+                selection_.setSelection(picks, selection_.activeObjectId());
+            }
+        } else if (!was_drag) {
+            // Click — apply press-time pick + modifiers.
+            if (tool_mode_ != ToolMode::None) {
+                emit surfacePickedInTool(e->pos().x(), e->pos().y(),
+                                         int(e->modifiers()));
+            } else {
+                const auto mods = e->modifiers();
+                if (mods & (Qt::ShiftModifier | Qt::ControlModifier)) {
+                    if (press_pick_id_ != 0) {
+                        // Toggle.  No-op on empty space so a stray
+                        // modifier-click doesn't blow away the set.
+                        selection_.toggleInSelection(press_pick_id_);
+                    }
+                } else {
+                    // Plain click — replace (or clear, for empty hit).
+                    selection_.setSelectedObjectId(press_pick_id_);
+                }
+            }
         }
+        // Drag from an object (no box select armed, movement > threshold)
+        // is intentionally a no-op — keeps stray drags from rewriting
+        // the selection.
+
+        box_select_armed_  = false;
+        box_select_active_ = false;
+        press_pick_id_     = 0;
     }
+
     const bool was_navigating = (active_button_ == Qt::MiddleButton);
     active_button_ = Qt::NoButton;
     if (was_navigating && pivot_indicator_visible_) {
@@ -3555,6 +3710,23 @@ void ViewportWindow::handleMouseMove(QMouseEvent* e) {
         last_mouse_pos_ = e->pos();
         return;
     }
+
+    // Promote an armed box-select to active once the cursor crosses the
+    // 5-px click threshold, then keep updating the rect for the overlay.
+    // last_mouse_pos_ stays at press position while armed so a fast tiny
+    // drag isn't classified as a click on release.
+    if (box_select_armed_ && active_button_ == Qt::LeftButton) {
+        if (!box_select_active_
+            && (e->pos() - box_select_start_pos_).manhattanLength() >= 5) {
+            box_select_active_ = true;
+        }
+        if (box_select_active_) {
+            box_select_current_pos_ = e->pos();
+            requestUpdate();
+        }
+        return;
+    }
+
     QPoint delta = e->pos() - last_mouse_pos_;
     last_mouse_pos_ = e->pos();
     if (active_button_ == Qt::MiddleButton) {
@@ -3697,7 +3869,8 @@ void ViewportWindow::setModelTransformation(uint32_t model_id,
 }
 
 void ViewportWindow::printSelectedObjectCoords() {
-    if (selected_object_id_ == 0) {
+    const uint32_t target = selection_.activeObjectId();
+    if (target == 0) {
         qInfo("printSelectedObjectCoords: no object selected");
         return;
     }
@@ -3710,7 +3883,7 @@ void ViewportWindow::printSelectedObjectCoords() {
         const ModelGpuData& m = kv.second;
         for (size_t i = 0; i < m.instances.size(); ++i) {
             const InstanceCpu& inst = m.instances[i];
-            if (inst.object_id != selected_object_id_) continue;
+            if (inst.object_id != target) continue;
 
             qInfo("Selected object %u (model %u, mesh %u, instance %zu):",
                   inst.object_id, inst.model_id, inst.mesh_id, i);
@@ -3776,7 +3949,7 @@ void ViewportWindow::printSelectedObjectCoords() {
         }
     }
     qInfo("printSelectedObjectCoords: object_id %u not found in any model",
-          selected_object_id_);
+          target);
 }
 
 bool ViewportWindow::readbackMeshTriangles(uint32_t model_id, uint32_t mesh_id,
diff --git a/src/ifcviewer/ViewportWindow.h b/src/ifcviewer/ViewportWindow.h
index 5b0e32ec21..8434c4fa25 100644
--- a/src/ifcviewer/ViewportWindow.h
+++ b/src/ifcviewer/ViewportWindow.h
@@ -34,6 +34,7 @@ QT_END_NAMESPACE
 
 #include <vector>
 #include <unordered_map>
+#include <unordered_set>
 #include <cstdint>
 #include <mutex>
 #include <memory>
@@ -46,6 +47,7 @@ QT_END_NAMESPACE
 #include "BvhAccel.h"
 #include "InstancedGeometry.h"
 #include "OverlayRenderer.h"
+#include "Selection.h"
 #include "SidecarCache.h"
 
 // Matches GL_DRAW_INDIRECT_BUFFER layout for glMultiDrawElementsIndirect.
@@ -306,8 +308,21 @@ public:
     void setModelCoordinateOperation(uint32_t model_id, const Eigen::Matrix4d& matrix_meters);
     void setModelTransformation(uint32_t model_id, const Eigen::Matrix4d& matrix_meters);
 
+    // Selection.  The viewport owns a SelectionState that tracks both
+    // the multi-set and the "active" (last single-clicked) id.  External
+    // callers (MainWindow / tree sync) drive selection through it.
+    SelectionState&       selection()       { return selection_; }
+    const SelectionState& selection() const { return selection_; }
+    // Convenience: replace the selection with {id} (or clear if id==0).
+    // Used by callers that have not yet been ported off the single-id API.
     void setSelectedObjectId(uint32_t id);
+
     uint32_t pickObjectAt(int x, int y);
+    // Render the pick pass and collect every distinct non-zero object_id
+    // covered by the pixels inside `rect` (logical coords, top-left origin).
+    // Used by the box-select drag.  Returns an empty set if rect is empty
+    // or off-surface.
+    std::unordered_set<uint32_t> picksInRect(const QRect& rect);
 
     // Extended pick: returns the object id, world-space hit point, and
     // world-space surface normal at (x, y).  Renders the same pick pass as
@@ -695,8 +710,23 @@ private:
     QElapsedTimer  fps_last_tick_;
     bool           fps_ignore_next_mouse_move_ = false;
 
-    // Selection
-    uint32_t selected_object_id_ = 0;
+    // Selection — set + active id + per-object_id flags SSBO (binding=3).
+    SelectionState selection_;
+
+    // LMB-press state.  press_pick_id_ caches the object hit at press
+    // time so the release path can apply click semantics without a
+    // second pick pass.  box_select_armed_ is set only when the press
+    // landed on empty space — a subsequent drag past the click
+    // threshold then promotes to box_select_active_, so a small wobble
+    // on a click doesn't accidentally box-select.  Modifiers captured
+    // at press-time decide commit semantics: plain replace, Shift add,
+    // Ctrl remove.
+    uint32_t  press_pick_id_          = 0;
+    bool      box_select_armed_       = false;
+    bool      box_select_active_      = false;
+    QPoint    box_select_start_pos_;
+    QPoint    box_select_current_pos_;
+    Qt::KeyboardModifiers box_select_press_mods_ = Qt::NoModifier;
 
     // Active measurement tool: see ToolMode / surfacePickedInTool.
     ToolMode tool_mode_              = ToolMode::None;