Simplify Models panel and dialog layout

Models panel: replace the manual resizeEvent column-sizing hack with
QHeaderView Stretch/Fixed modes, re-applied via sectionCountChanged so
they survive the model rebuilds that QHeaderView resets them on.

Dialog: only wrap the body in a QScrollArea when scrollable, mirroring
Panel. The scroll area caps its sizeHint at 36x24 cells, which turned
wide fixed-size dialog content into spurious scrollbars.

Add Model dialog: reserve a stable, font-metrics-measured height for the
hover description so longer text never reflows the buttons; regroup the
buttons into LOCAL / CLOUD / TOOLS.

Buttons: move the trailing-separator decision out of makeButtonGroup
into a new addButtonGroups row builder, so the last group in a row
never draws a dangling divider.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/bonsaiviewer/components/Buttons.cpp

@@ -22,6 +22,7 @@
 
 #include "SvgIcon.h"
 
+#include <QBoxLayout>
 #include <QFrame>
 #include <QHBoxLayout>
 #include <QLabel>
@@ -47,11 +48,9 @@ QToolButton* makeButton(const QString& text,
 QWidget* makeButtonGroup(const QString& title,
                          const QList<QToolButton*>& buttons,
                          QWidget* parent,
-                         bool trailing_separator,
                          int vertical_spacing) {
     auto* group = new QFrame(parent);
     group->setObjectName("ribbonGroup");
-    group->setProperty("separator", trailing_separator);
 
     auto* group_layout = new QVBoxLayout(group);
     group_layout->setContentsMargins(8, 6, 8, 4);
@@ -74,4 +73,13 @@ QWidget* makeButtonGroup(const QString& title,
     return group;
 }
 
+void addButtonGroups(QBoxLayout* row, const QList<QWidget*>& groups) {
+    for (int i = 0; i < groups.size(); ++i) {
+        // The divider belongs *between* groups; the last group never draws a
+        // trailing one. The stylesheet keys off this dynamic property.
+        groups[i]->setProperty("separator", i + 1 < groups.size());
+        row->addWidget(groups[i]);
+    }
+}
+
 } // namespace bonsaiviewer::components::buttons

src/bonsaiviewer/components/Buttons.h

@@ -23,6 +23,7 @@
 
 #include <QList>
 
+class QBoxLayout;
 class QToolButton;
 class QWidget;
 
@@ -35,9 +36,13 @@ QToolButton* makeButton(const QString& text,
 QWidget* makeButtonGroup(const QString& title,
                          const QList<QToolButton*>& buttons,
                          QWidget* parent,
-                         bool trailing_separator = true,
                          int vertical_spacing = 4);
 
+// Adds button groups to a ribbon row, drawing a vertical divider between
+// adjacent groups but never after the last one. Centralising the decision
+// here means a row can't end up with a dangling trailing separator.
+void addButtonGroups(QBoxLayout* row, const QList<QWidget*>& groups);
+
 } // namespace bonsaiviewer::components::buttons
 
 #endif

src/bonsaiviewer/components/Dialog.cpp

@@ -49,19 +49,27 @@ Dialog::Dialog(QWidget* parent, bool scrollable)
                                      style::metrics::section_body_padding);
     frame_layout->setSpacing(0);
 
-    auto* scroll = new QScrollArea(frame);
-    scroll->setWidgetResizable(true);
-    scroll->setFrameShape(QFrame::NoFrame);
-
-    auto* scroll_body = new QWidget(scroll);
-    scroll_body->setObjectName("panelScrollBody");
-    body_layout_ = new QVBoxLayout(scroll_body);
+    // A non-scrollable dialog must not be wrapped in a QScrollArea. The scroll
+    // area caps its own sizeHint at 36x24 character cells, and dialogs size
+    // themselves with QLayout::SetFixedSize — so any content wider/taller than
+    // that cap is turned into scrollbars instead of growing the dialog. Only
+    // use a scroll area when scrolling is actually wanted (mirrors Panel).
+    auto* body = new QWidget(frame);
+    body->setObjectName("panelScrollBody");
+    body_layout_ = new QVBoxLayout(body);
     body_layout_->setContentsMargins(0, 0, 0, 0);
     body_layout_->setSpacing(style::metrics::section_body_padding);
     body_layout_->setAlignment(Qt::AlignTop);
 
-    scroll->setWidget(scroll_body);
-    frame_layout->addWidget(scroll, scrollable ? 1 : 0);
+    if (scrollable) {
+        auto* scroll = new QScrollArea(frame);
+        scroll->setWidgetResizable(true);
+        scroll->setFrameShape(QFrame::NoFrame);
+        scroll->setWidget(body);
+        frame_layout->addWidget(scroll, 1);
+    } else {
+        frame_layout->addWidget(body);
+    }
 
     auto* footer = new QWidget(frame);
     footer_layout_ = new QVBoxLayout(footer);