Pane tabs: Scroll entire new tab into view (#36827)

The state of the child bounds is not up-to-date when `scroll_to_item`
gets triggered, causing the new tab to not scroll completely into view.

Closes #36317 

Release Notes:

- Fix an issue where a new tab is only partially visible on creation.

crates/collab/src/tests/integration_tests.rs

@@ -6514,14 +6514,8 @@ async fn test_right_click_menu_behind_collab_panel(cx: &mut TestAppContext) {
     cx.simulate_keystrokes("cmd-n cmd-n cmd-n");
     cx.update(|window, _cx| window.refresh());
 
-    let tab_bounds = cx.debug_bounds("TAB-2").unwrap();
     let new_tab_button_bounds = cx.debug_bounds("ICON-Plus").unwrap();
 
-    assert!(
-        tab_bounds.intersects(&new_tab_button_bounds),
-        "Tab should overlap with the new tab button, if this is failing check if there's been a redesign!"
-    );
-
     cx.simulate_event(MouseDownEvent {
         button: MouseButton::Right,
         position: new_tab_button_bounds.center(),