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/terminal_view/src/terminal_element.rs

@@ -541,12 +541,12 @@ impl TerminalElement {
 }
 
 impl Element for TerminalElement {
-    type BeforeLayout = ();
-    type AfterLayout = LayoutState;
+    type RequestLayoutState = ();
+    type PrepaintState = LayoutState;
 
-    fn before_layout(&mut self, cx: &mut ElementContext) -> (LayoutId, Self::BeforeLayout) {
+    fn request_layout(&mut self, cx: &mut ElementContext) -> (LayoutId, Self::RequestLayoutState) {
         self.interactivity.occlude_mouse();
-        let layout_id = self.interactivity.before_layout(cx, |mut style, cx| {
+        let layout_id = self.interactivity.request_layout(cx, |mut style, cx| {
             style.size.width = relative(1.).into();
             style.size.height = relative(1.).into();
             let layout_id = cx.request_layout(&style, None);
@@ -556,14 +556,14 @@ impl Element for TerminalElement {
         (layout_id, ())
     }
 
-    fn after_layout(
+    fn prepaint(
         &mut self,
         bounds: Bounds<Pixels>,
-        _: &mut Self::BeforeLayout,
+        _: &mut Self::RequestLayoutState,
         cx: &mut ElementContext,
-    ) -> Self::AfterLayout {
+    ) -> Self::PrepaintState {
         self.interactivity
-            .after_layout(bounds, bounds.size, cx, |_, _, hitbox, cx| {
+            .prepaint(bounds, bounds.size, cx, |_, _, hitbox, cx| {
                 let hitbox = hitbox.unwrap();
                 let settings = ThemeSettings::get_global(cx).clone();
 
@@ -669,7 +669,7 @@ impl Element for TerminalElement {
                         .id("terminal-element")
                         .tooltip(move |cx| Tooltip::text(hovered_word.word.clone(), cx))
                         .into_any_element();
-                    element.layout(offset, bounds.size.into(), cx);
+                    element.prepaint_as_root(offset, bounds.size.into(), cx);
                     element
                 });
 
@@ -775,8 +775,8 @@ impl Element for TerminalElement {
     fn paint(
         &mut self,
         bounds: Bounds<Pixels>,
-        _: &mut Self::BeforeLayout,
-        layout: &mut Self::AfterLayout,
+        _: &mut Self::RequestLayoutState,
+        layout: &mut Self::PrepaintState,
         cx: &mut ElementContext<'_>,
     ) {
         cx.paint_quad(fill(bounds, layout.background_color));