Remove unconditional invalidation when calling mouse region handlers

We want invalidation to opt-in as much as possible.
If you want a view to re-render, you need to call `cx.notify`.

crates/gpui/src/elements/list.rs

@@ -558,6 +558,8 @@ impl StateInner {
             let visible_range = self.visible_range(height, scroll_top);
             self.scroll_handler.as_mut().unwrap()(visible_range, cx);
         }
+
+        cx.notify();
     }
 
     fn scroll_top(&self, logical_scroll_top: &ListOffset) -> f32 {