MacViews: Reattach children of native windows when they are exposed.

ui/views/cocoa/widget_owner_nswindow_adapter.mm

Index: ui/views/cocoa/widget_owner_nswindow_adapter.mm
diff --git a/ui/views/cocoa/widget_owner_nswindow_adapter.mm b/ui/views/cocoa/widget_owner_nswindow_adapter.mm
index 63cc4468d95449b86bbb6a2a2a8ab06cf9dd2c54..523453591a6be0e0b060321a25ce5fed27a37df4 100644
--- a/ui/views/cocoa/widget_owner_nswindow_adapter.mm
+++ b/ui/views/cocoa/widget_owner_nswindow_adapter.mm
@@ -21,6 +21,7 @@
 }
 - (instancetype)initWithAdapter:(views::WidgetOwnerNSWindowAdapter*)adapter;
 - (void)windowWillClose:(NSNotification*)notification;
+- (void)windowDidChangeOcclusionState:(NSNotification*)notification;
 @end
 
 @implementation WidgetOwnerNSWindowAdapterBridge
@@ -35,6 +36,10 @@
   adapter_->OnWindowWillClose();
 }
 
+- (void)windowDidChangeOcclusionState:(NSNotification*)notification {
+  adapter_->OnWindowDidChangeOcclusionState();
+}
+
 @end
 
 namespace views {
@@ -59,6 +64,12 @@ WidgetOwnerNSWindowAdapter::WidgetOwnerNSWindowAdapter(
          selector:@selector(windowWillClose:)
              name:NSWindowWillCloseNotification
            object:anchor_window_];
+
+  [[NSNotificationCenter defaultCenter]

[karandeepb, 2016/10/27 03:21:17]

Maybe add a comment explaining why we need to observe changes to the occlusion
state.

[tapted, 2016/10/27 04:06:48]

Done.

+      addObserver:observer_bridge_
+         selector:@selector(windowDidChangeOcclusionState:)
+             name:NSWindowDidChangeOcclusionStateNotification
+           object:anchor_window_];
 }
 
 void WidgetOwnerNSWindowAdapter::OnWindowWillClose() {
@@ -80,6 +91,30 @@ void WidgetOwnerNSWindowAdapter::OnWindowWillClose() {
   DCHECK(![child_window delegate]);
 }
 
+void WidgetOwnerNSWindowAdapter::OnWindowDidChangeOcclusionState() {
+  // The adapter only needs to handle a parent "show", since the only way it
+  // should be hidden is via -[NSApp hide], and all BridgedNativeWidgets
+  // subscribe to NSApplicationDidHideNotification already.
+  if (![anchor_window_ isVisible])

[karandeepb, 2016/10/27 03:21:17]

Just to clarify, the parent-child relationship breaks when the window is hidden.
It does not break when the window is not hidden but only occluded, right?

And what we want to do is to reattach the child when the parent window becomes
"visible" (as opposed to just being non-occluded)? Buy since there is no proper
visibility change notification, we are using occlusion state.

[tapted, 2016/10/27 04:06:48]

Yup. AppKit fails to update zorder/position for child windows after an orderOut,
so we intentionally break the relationship in those cases. But it's fine just
for occlusion.
Yep - exactly. There is a WindowDidExpose notification but it doesn't work.
Visibility changes on Mac are a pain, but occlusions works pretty well. It came
in 10.9, so lucky we don't support further back than that :)

+    return;
+
+  if (child_->window_visible()) {
+    DCHECK(child_->wants_to_be_visible());

[karandeepb, 2016/10/27 03:21:17]

Should we DCHECK([child->ns_window() parentWindow]);

[tapted, 2016/10/27 04:06:48]

Done.

+    return;
+  }
+
+  if (!child_->wants_to_be_visible())
+    return;
+

[karandeepb, 2016/10/27 03:21:17]

So now the parent is visible, and the child is hidden but wants to be visible
(it is not visible because it had a hidden parent). Can we add a DCHECK here
that the occlusion state of the anchor window has changed to
NSWindowOcclusionStateVisible?

[tapted, 2016/10/27 04:06:48]

I don't think this is the same. The parent window can be "visible", but fully
occluded -- we don't hide child widgets in that situation, so we shouldn't block
re-showing them either. And since occlusion is generated by the window server,
there's possible race conditions that could mean a window could be
fully-occluded the first time it becomes "visible".

+  // The parent relationship should have been removed when the child was hidden.
+  DCHECK(![child_->ns_window() parentWindow]);

[karandeepb, 2016/10/27 03:21:17]

Can this DCHECK move before the preceding if condition?

[tapted, 2016/10/27 04:06:48]

Done.

+  [child_->ns_window() orderWindow:NSWindowAbove
+                        relativeTo:[anchor_window_ windowNumber]];
+
+  // Ordering the window should add back the relationship.
+  DCHECK([child_->ns_window() parentWindow]);

[karandeepb, 2016/10/27 03:21:17]

DCHECK(child_->window_visible())?

[tapted, 2016/10/27 04:06:48]

Done.

+}
+
 NSWindow* WidgetOwnerNSWindowAdapter::GetNSWindow() {
   return anchor_window_;
 }