Fix context_stack race in KeyContextView (#29324)

cc @notpeter

Before this change we used our own copy of `cx.key_context()` when
matching.
This led to races where the context queried could be either before (or
after) the
context used in dispatching.

To avoid the race, gpui now passes out the context stack actually used
instead.

Release Notes:

- Fixed a bug where the Key Context View could show the incorrect
context,
  causing confusing results.

crates/language_tools/src/key_context_view.rs

@@ -41,17 +41,17 @@ struct KeyContextView {
 
 impl KeyContextView {
     pub fn new(window: &mut Window, cx: &mut Context<Self>) -> Self {
-        let sub1 = cx.observe_keystrokes(|this, e, window, cx| {
+        let sub1 = cx.observe_keystrokes(|this, e, _, cx| {
             let mut pending = this.pending_keystrokes.take().unwrap_or_default();
             pending.push(e.keystroke.clone());
             let mut possibilities = cx.all_bindings_for_input(&pending);
             possibilities.reverse();
-            this.context_stack = window.context_stack();
             this.last_keystrokes = Some(
                 json!(pending.iter().map(|p| p.unparse()).join(" "))
                     .to_string()
                     .into(),
             );
+            this.context_stack = e.context_stack.clone();
             this.last_possibilities = possibilities
                 .into_iter()
                 .map(|binding| {
@@ -89,6 +89,7 @@ impl KeyContextView {
                     )
                 })
                 .collect();
+            cx.notify();
         });
         let sub2 = cx.observe_pending_input(window, |this, window, cx| {
             this.pending_keystrokes = window
@@ -237,7 +238,7 @@ impl Render for KeyContextView {
                     .mt_8(),
             )
             .children({
-                window.context_stack().into_iter().enumerate().map(|(i, context)| {
+                self.context_stack.iter().enumerate().map(|(i, context)| {
                     let primary = context.primary().map(|e| e.key.clone()).unwrap_or_default();
                     let secondary = context
                         .secondary()