fix: Keep workspace stack count fresh instead of hiding it on Apple (#9885)

Screen readers read the workspace's region label when its contents are
focused, but updateAriaLabel only ran on the workspace node's own focus.
When a child block took focus on first entry the label was never
refreshed, so a stale stack count (e.g. "0 stacks") was announced. The
previous workaround dropped the count entirely on Apple.

Refresh the label in onTreeFocus, which fires whenever focus enters the
workspace tree (including via a child), before the element is focused, so
the announced count is current. This lets us restore the informative
stack count on all platforms.

Author: microbit-matt-hillsdon

packages/blockly/core/workspace_svg.ts

@@ -745,17 +745,6 @@ export class WorkspaceSvg
    * Updates the label on the workspace to reflect the number of top-level stacks in the workspace.
    */
   private updateAriaLabel() {
-    if (userAgent.APPLE) {
-      // VoiceOver is reading this label inappropriately, so don't show the
-      // stack count because it might be inaccurate.
-      // https://github.com/RaspberryPiFoundation/blockly/issues/9885
-      aria.setState(
-        this.svgGroup_,
-        aria.State.LABEL,
-        Msg['WORKSPACE_LABEL_PLAIN'],
-      );
-      return;
-    }
     const numStacks = this.getTopBlocks(false).length;
     if (numStacks == 1) {
       aria.setState(
@@ -2975,7 +2964,14 @@ export class WorkspaceSvg
   onTreeFocus(
     _node: IFocusableNode,
     _previousTree: IFocusableTree | null,
-  ): void {}
+  ): void {
+    // Screen readers read this label as the enclosing region when workspace
+    // contents are focused, so refresh it here to keep the stack count from
+    // going stale.
+    if (!this.isFlyout && !this.isMutator) {
+      this.updateAriaLabel();
+    }
+  }
 
   /** See IFocusableTree.onTreeBlur. */
   onTreeBlur(nextTree: IFocusableTree | null): void {