Picture Format: a resize alone brings the Picture Styles row to the new width

The ribbon now asks for one more frame when a tab's spare room changes by
more than half a point, so the row is sized for the new width even when no
input follows the resize. The test resizes 1440 to 1920 and back and checks
the harness settles.

crates/ui-egui/src/ribbon.rs

@@ -237,7 +237,17 @@ pub fn show(app: &mut SlideApp, ui: &mut Ui) {
                 });
             });
         let r = scroll.inner_rect;
-        ui.ctx().data_mut(|d| d.insert_temp(spare_room_id(&tab), r.width() - scroll.content_size.x));
+        // A gallery sized from this reads it on the next frame, so a change (a resize, a tab
+        // shown for the first time) asks for one; within half a point it is the same layout.
+        let spare = r.width() - scroll.content_size.x;
+        let moved = ui.ctx().data_mut(|d| {
+            let was = d.get_temp::<f32>(spare_room_id(&tab));
+            d.insert_temp(spare_room_id(&tab), spare);
+            was.is_none_or(|was| (was - spare).abs() > 0.5)
+        });
+        if moved {
+            ui.ctx().request_repaint();
+        }
         let overflow = (scroll.content_size.x - r.width()).max(0.0);
         let offset = scroll.state.clamped_offset().x;
         // Record the rendered offset for `InputTarget`: the same window point hits a different

crates/ui-egui/src/ribbon/picture_styles_tests.rs

@@ -89,3 +89,19 @@ fn more_picture_styles_lists_every_style_and_applies_the_one_clicked() {
     assert_eq!(doc.history.undo.len(), undo + 1, "one undo step");
     assert!(h.query_by_label(last).is_none(), "the menu closes after a pick");
 }
+
+#[test]
+fn a_resize_alone_settles_the_styles_row() {
+    // No pointer event follows the resize, so only the ribbon's own repaint request can bring the
+    // row to the new width.
+    let (mut h, _) = picture_format(1440.0);
+    h.run_ok();
+    assert_eq!(inline_styles(&h), 2);
+    h.set_size(egui::vec2(1920.0, 900.0));
+    assert!(h.run_ok().is_some(), "the ribbon keeps asking for frames after a resize");
+    assert_eq!(inline_styles(&h), 8, "the row still has the 1440 px count after growing to 1920 px");
+    assert!(h.query_by_label("Scroll ribbon right").is_none());
+    h.set_size(egui::vec2(1440.0, 900.0));
+    h.run_ok();
+    assert_eq!(inline_styles(&h), 2, "the row still has the 1920 px count after shrinking to 1440 px");
+}