Introduce autoscroll support for elements (#10889)

This pull request introduces the new
`ElementContext::request_autoscroll(bounds)` and
`ElementContext::take_autoscroll()` methods in GPUI. These new APIs
enable container elements such as `List` to change their scroll position
if one of their children requested an autoscroll. We plan to use this in
the revamped assistant.

As a drive-by, we also:

- Renamed `Element::before_layout` to `Element::request_layout`
- Renamed `Element::after_layout` to `Element::prepaint`
- Introduced a new `List::splice_focusable` method to splice focusable
elements into the list, which enables rendering offscreen elements that
are focused.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/workspace/src/pane_group.rs

@@ -792,13 +792,13 @@ mod element {
     }
 
     impl Element for PaneAxisElement {
-        type BeforeLayout = ();
-        type AfterLayout = PaneAxisLayout;
+        type RequestLayoutState = ();
+        type PrepaintState = PaneAxisLayout;
 
-        fn before_layout(
+        fn request_layout(
             &mut self,
             cx: &mut ui::prelude::ElementContext,
-        ) -> (gpui::LayoutId, Self::BeforeLayout) {
+        ) -> (gpui::LayoutId, Self::RequestLayoutState) {
             let mut style = Style::default();
             style.flex_grow = 1.;
             style.flex_shrink = 1.;
@@ -808,10 +808,10 @@ mod element {
             (cx.request_layout(&style, None), ())
         }
 
-        fn after_layout(
+        fn prepaint(
             &mut self,
             bounds: Bounds<Pixels>,
-            _state: &mut Self::BeforeLayout,
+            _state: &mut Self::RequestLayoutState,
             cx: &mut ElementContext,
         ) -> PaneAxisLayout {
             let dragged_handle = cx.with_element_state::<Rc<RefCell<Option<usize>>>, _>(
@@ -872,7 +872,8 @@ mod element {
                     size: child_size,
                 };
                 bounding_boxes.push(Some(child_bounds));
-                child.layout(origin, child_size.into(), cx);
+                child.layout_as_root(child_size.into(), cx);
+                child.prepaint_at(origin, cx);
 
                 origin = origin.apply_along(self.axis, |val| val + child_size.along(self.axis));
                 layout.children.push(PaneAxisChildLayout {
@@ -897,8 +898,8 @@ mod element {
         fn paint(
             &mut self,
             bounds: gpui::Bounds<ui::prelude::Pixels>,
-            _: &mut Self::BeforeLayout,
-            layout: &mut Self::AfterLayout,
+            _: &mut Self::RequestLayoutState,
+            layout: &mut Self::PrepaintState,
             cx: &mut ui::prelude::ElementContext,
         ) {
             for child in &mut layout.children {