Add a new Hooks element to invoke a callback before layout

This is useful to cap the width of sidebars when dragging the
resize handles beyond the maximum bounds of the sidebar.

zed/src/workspace/sidebar.rs

@@ -112,12 +112,21 @@ impl Sidebar {
             if matches!(self.side, Side::Right) {
                 container.add_child(self.render_resize_handle(settings, cx));
             }
+
+            let width = self.width.clone();
             container.add_child(
                 Flexible::new(
                     1.,
-                    ConstrainedBox::new(ChildView::new(active_item.id()).boxed())
-                        .with_max_width(*self.width.borrow())
-                        .boxed(),
+                    Hooks::new(
+                        ConstrainedBox::new(ChildView::new(active_item.id()).boxed())
+                            .with_max_width(*self.width.borrow())
+                            .boxed(),
+                    )
+                    .on_before_layout(move |constraint, _| {
+                        let mut width = width.borrow_mut();
+                        *width = width.min(constraint.max.x());
+                    })
+                    .boxed(),
                 )
                 .boxed(),
             );