[a11y] fixes overlayPortals not showing VoiceControl labels (#164754)

Fixes flutter/flutter#157753
Even when a `uiaccessibilityelement` is marked as
`accessibilityRespondsToUserInteraction`, if it's spatially far away
enough from its parent's `accessibilityFrame`, a VoiceControl label will
not be created for it. In this PR, I set the parent
`SemanticObjectContainer`'s `accessibilityFrame` so it is the minimum
rect that will cover all it's children.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

Author: LouiseHsu

engine/src/flutter/shell/platform/darwin/ios/framework/Source/SemanticsObject.mm

@@ -941,7 +941,12 @@ - (BOOL)isAccessibilityElement {
 }
 
 - (CGRect)accessibilityFrame {
-  return self.semanticsObject.accessibilityFrame;
+  // For OverlayPortals, the child element is sometimes outside the bounds of the parent
+  // Even if it's marked accessible, VoiceControl labels will not appear if it's too
+  // spatially distant. Set the frame to be the max screen size so all children are guaraenteed
+  // to be contained.
+
+  return UIScreen.mainScreen.bounds;
 }
 
 - (id)accessibilityContainer {

engine/src/flutter/shell/platform/darwin/ios/framework/Source/SemanticsObjectTest.mm

@@ -614,6 +614,41 @@ - (void)testFlutterSemanticsObjectMergeTooltipToLabel {
   XCTAssertTrue([object.accessibilityLabel isEqualToString:@"label\ntooltip"]);
 }
 
+- (void)testSemanticsObjectContainerAccessibilityFrameCoversEntireScreen {
+  flutter::testing::MockAccessibilityBridge* mock = new flutter::testing::MockAccessibilityBridge();
+  mock->isVoiceOverRunningValue = true;
+  fml::WeakPtrFactory<flutter::AccessibilityBridgeIos> factory(mock);
+  fml::WeakPtr<flutter::AccessibilityBridgeIos> bridge = factory.GetWeakPtr();
+
+  flutter::SemanticsNode parent;
+  parent.id = 0;
+  parent.actions = static_cast<int32_t>(flutter::SemanticsAction::kTap);
+
+  flutter::SemanticsNode child;
+  child.id = 1;
+  child.actions = static_cast<int32_t>(flutter::SemanticsAction::kTap);
+  child.rect = SkRect::MakeXYWH(0, 0, 100, 100);
+  parent.childrenInTraversalOrder.push_back(1);
+
+  FlutterSemanticsObject* parentObject = [[FlutterSemanticsObject alloc] initWithBridge:bridge
+                                                                                    uid:0];
+  [parentObject setSemanticsNode:&parent];
+
+  FlutterSemanticsObject* childObject = [[FlutterSemanticsObject alloc] initWithBridge:bridge
+                                                                                   uid:1];
+  [childObject setSemanticsNode:&child];
+
+  parentObject.children = @[ childObject ];
+  [parentObject accessibilityBridgeDidFinishUpdate];
+  [childObject accessibilityBridgeDidFinishUpdate];
+
+  SemanticsObjectContainer* container =
+      static_cast<SemanticsObjectContainer*>(parentObject.accessibilityContainer);
+
+  XCTAssertTrue(childObject.accessibilityRespondsToUserInteraction);
+  XCTAssertTrue(CGRectEqualToRect(container.accessibilityFrame, UIScreen.mainScreen.bounds));
+}
+
 - (void)testFlutterSemanticsObjectAttributedStringsDoNotCrashWhenEmpty {
   flutter::testing::MockAccessibilityBridge* mock = new flutter::testing::MockAccessibilityBridge();
   mock->isVoiceOverRunningValue = true;