Fix the problem that black background window might occasionally appear in front on Windows

When a new panel is added to a stack, the following things occur:
1) The panel is initially created as always on top on Windows.
2) The panel is changed to non-always-on-top.
3) The background stack window is made to the owner of the new panel window.
4) The background stack window is set to behind the new panel window in z-order.

Occasionally, step 2 might be run after step 4. When this occurs, after step 1, 3 and 4, the background stack window will automatically obtain the always-on-top state since the panel window behind which is set to is always-on-top. For most of time, running step 2 after 4 will remove the always-on-top state for both new panel window and background stack window. However, if chrome is not foreground application, Windows system might fail to deprive the always-on-top state for the background stack window.

The fix is to create the panel window with always-on-top state depending on its collection.

BUG=241224
TEST=Manual test by following the bug report

Review URL: https://chromiumcodereview.appspot.com/14956006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@200788 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 4 insertions, 1 deletions

diff --git a/chrome/browser/ui/panels/detached_panel_collection.cc b/chrome/browser/ui/panels/detached_panel_collection.cc
index 3437237..d525dac 100644
--- a/chrome/browser/ui/panels/detached_panel_collection.cc
+++ b/chrome/browser/ui/panels/detached_panel_collection.cc
@@ -190,6 +190,10 @@ bool DetachedPanelCollection::IsPanelMinimized(const Panel* panel) const {
   return false;
 }
 
+bool DetachedPanelCollection::UsesAlwaysOnTopPanels() const {
+  return false;
+}
+
 void DetachedPanelCollection::SavePanelPlacement(Panel* panel) {
   DCHECK(!saved_panel_placement_.panel);
   saved_panel_placement_.panel = panel;
@@ -234,7 +238,6 @@ void DetachedPanelCollection::UpdatePanelOnCollectionChange(Panel* panel) {
   panel->set_attention_mode(
       static_cast<Panel::AttentionMode>(Panel::USE_PANEL_ATTENTION |
                                         Panel::USE_SYSTEM_ATTENTION));
-  panel->SetAlwaysOnTop(false);
   panel->ShowShadow(true);
   panel->EnableResizeByMouse(true);
   panel->UpdateMinimizeRestoreButtonVisibility();