Fix #808: the toolbar's scroll bar stays a slim line

The toolbar and the dock's icon column scroll when the window is too
short for them. Their scroll bar was egui's floating one, which widens to
10 pt while scrolled or dragged and covered the group labels and the
icons' right edge. It now stays at most 4 pt wide (2 pt at rest), at the
column's edge.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/ui-egui/src/widgets.rs

@@ -785,8 +785,20 @@ pub fn menu_scroll<R>(ui: &mut Ui, add_contents: impl FnOnce(&mut Ui) -> R) -> R
 /// A column of tool or panel buttons (the toolbar, the dock's icon column) that scrolls when the
 /// window is too short for it: with the mouse wheel, or the scroll bar shown while the pointer is
 /// over it.
+/// Its scroll bar stays a slim line at the edge, even while scrolled or dragged, so it never covers
+/// the buttons (#808).
 pub fn strip_scroll<R>(ui: &mut Ui, id: &str, add_contents: impl FnOnce(&mut Ui) -> R) -> R {
-    egui::ScrollArea::vertical().id_salt(id).auto_shrink([false, true]).show(ui, add_contents).inner
+    ui.scope(|ui| {
+        ui.spacing_mut().scroll = egui::style::ScrollStyle {
+            bar_width: 4.0,
+            floating_width: 2.0,
+            bar_inner_margin: 0.0,
+            bar_outer_margin: 1.0,
+            ..egui::style::ScrollStyle::floating()
+        };
+        egui::ScrollArea::vertical().id_salt(id).auto_shrink([false, true]).show(ui, add_contents).inner
+    })
+    .inner
 }
 
 /// Whether the blend-mode list draws a separator above `BlendMode::ALL[i]`: where a