workspace: Add command to reset pane sizes (#59046)

## Summary

Exposes the existing recursive pane-size reset as a command-palette
action, so it can be invoked outside of vim mode.

The center pane grid's sizes can already be evened out by
double-clicking a divider, or via `vim::ResetPaneSizes` (`ctrl-w =`)
when vim mode is on, but there is no general action for it — so users
not in vim mode have no discoverable way to undo a drifted split layout
in one step.

## How it works

`workspace::ResetPaneSizes` (palette: "workspace: reset pane sizes")
calls the existing `Workspace::reset_pane_sizes`, which resets every
`PaneAxis` in the center pane group to equal flexes, recursing into
nested splits. The split structure is preserved — no panes are added,
removed, or rearranged, only their sizes are equalized. This is the same
path the vim `ctrl-w =` binding and the divider double-click already
use.

## Out of scope

- Docks (left/right/bottom panels) — they keep their existing `Reset
Active Dock Size` / `Reset Open Docks Size` actions.
- The terminal panel's internal splits — it is a separate panel, not
part of the center pane group.
- No default keybinding (palette-only); vim users keep `ctrl-w =`.

## Testing

Added `test_reset_pane_sizes`: builds a nested split (a horizontal axis
of three panes whose last child is a vertical split), skews every axis's
flexes, dispatches the action, and asserts every axis returns to uniform
sizes.

Release Notes:

- Added a `workspace: reset pane sizes` command that equalizes the sizes
of all panes in the center group

crates/workspace/src/workspace.rs

@@ -303,6 +303,8 @@ actions!(
         ResetActiveDockSize,
         /// Resets all open docks to their default sizes.
         ResetOpenDocksSize,
+        /// Resets all panes in the center group to equal sizes, preserving the split layout.
+        ResetPaneSizes,
         /// Reloads the application
         Reload,
         /// Formats and saves the current file, regardless of the format_on_save setting.
@@ -7651,6 +7653,11 @@ impl Workspace {
                     }
                 },
             ))
+            .on_action(cx.listener(
+                |workspace: &mut Workspace, _: &ResetPaneSizes, _window, cx| {
+                    workspace.reset_pane_sizes(cx);
+                },
+            ))
             .on_action(cx.listener(
                 |workspace: &mut Workspace, act: &IncreaseActiveDockSize, window, cx| {
                     adjust_active_dock_size_by_px(
@@ -12771,6 +12778,74 @@ mod tests {
         });
     }
 
+    #[gpui::test]
+    async fn test_reset_pane_sizes(cx: &mut gpui::TestAppContext) {
+        init_test(cx);
+        let fs = FakeFs::new(cx.executor());
+        let project = Project::test(fs, [], cx).await;
+        let (multi_workspace, cx) =
+            cx.add_window_view(|window, cx| MultiWorkspace::test_new(project, window, cx));
+        let workspace = multi_workspace.read_with(cx, |mw, _| mw.workspace().clone());
+
+        // A horizontal split of three panes whose last child is itself a vertical
+        // split, so equalizing has to recurse into the nested axis.
+        workspace.update_in(cx, |workspace, window, cx| {
+            let item = cx.new(|cx| {
+                TestItem::new(cx).with_project_items(&[TestProjectItem::new(1, "1.txt", cx)])
+            });
+            workspace.add_item_to_active_pane(Box::new(item), None, true, window, cx);
+            workspace.split_pane(
+                workspace.active_pane().clone(),
+                SplitDirection::Right,
+                window,
+                cx,
+            );
+            workspace.split_pane(
+                workspace.active_pane().clone(),
+                SplitDirection::Right,
+                window,
+                cx,
+            );
+            workspace.split_pane(
+                workspace.active_pane().clone(),
+                SplitDirection::Down,
+                window,
+                cx,
+            );
+        });
+
+        let nested_axis = |workspace: &Workspace| {
+            let Member::Axis(top) = &workspace.center.root else {
+                panic!("expected the center to be a split axis");
+            };
+            let nested = top
+                .members
+                .iter()
+                .find_map(|member| match member {
+                    Member::Axis(axis) => Some(axis.clone()),
+                    Member::Pane(_) => None,
+                })
+                .expect("expected a nested split axis");
+            (top.clone(), nested)
+        };
+
+        // Skew every axis away from uniform sizes.
+        workspace.update(cx, |workspace, _| {
+            let (top, nested) = nested_axis(workspace);
+            *top.flexes.lock() = vec![1.6, 0.7, 0.7];
+            *nested.flexes.lock() = vec![1.3, 0.7];
+        });
+
+        cx.run_until_parked();
+        cx.dispatch_action(ResetPaneSizes);
+
+        workspace.update(cx, |workspace, _| {
+            let (top, nested) = nested_axis(workspace);
+            assert_eq!(*top.flexes.lock(), vec![1.0; top.members.len()]);
+            assert_eq!(*nested.flexes.lock(), vec![1.0; nested.members.len()]);
+        });
+    }
+
     #[gpui::test]
     async fn test_toggle_docks_and_panels(cx: &mut gpui::TestAppContext) {
         init_test(cx);