Bug 1575635 - [1/1] Only use parent window's fullscreen state r=win-reviewers,mhowell

When we open a new window on top of a fullscreen window, we deliberately
make that window "normal" (non-fullscreen, non-maximized), which was
documented as having been "a standard [W]indows convention" [1].
Chromium does the same (with fiddly differences involving the window
size).

Unfortunately, we do this by checking if there are _any_ fullscreen
windows open -- which produces false positives on multimonitor or
multi-virtual-desktop setups.

Therefore, adjust the code to match the original comment, and only take
the parent window into account. This seems likely to better match user
expectations.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=575195#c5

Differential Revision: https://phabricator.services.mozilla.com/D197823

Author: selenography

xpfe/appshell/nsAppShellService.cpp

@@ -503,37 +503,6 @@ uint32_t nsAppShellService::CalculateWindowZLevel(nsIAppWindow* aParent,
   return zLevel;
 }
 
-#ifdef XP_WIN
-/*
- * Checks to see if any existing window is currently in fullscreen mode.
- */
-static bool CheckForFullscreenWindow() {
-  nsCOMPtr<nsIWindowMediator> wm(do_GetService(NS_WINDOWMEDIATOR_CONTRACTID));
-  if (!wm) return false;
-
-  nsCOMPtr<nsISimpleEnumerator> windowList;
-  wm->GetAppWindowEnumerator(nullptr, getter_AddRefs(windowList));
-  if (!windowList) return false;
-
-  for (;;) {
-    bool more = false;
-    windowList->HasMoreElements(&more);
-    if (!more) return false;
-
-    nsCOMPtr<nsISupports> supportsWindow;
-    windowList->GetNext(getter_AddRefs(supportsWindow));
-    nsCOMPtr<nsIBaseWindow> baseWin(do_QueryInterface(supportsWindow));
-    if (baseWin) {
-      nsCOMPtr<nsIWidget> widget;
-      baseWin->GetMainWidget(getter_AddRefs(widget));
-      if (widget && widget->SizeMode() == nsSizeMode_Fullscreen) {
-        return true;
-      }
-    }
-  }
-}
-#endif
-
 /*
  * Just do the window-making part of CreateTopLevelWindow
  */
@@ -554,7 +523,13 @@ nsresult nsAppShellService::JustCreateTopWindow(
   // If the parent is currently fullscreen, tell the child to ignore persisted
   // full screen states. This way new browser windows open on top of fullscreen
   // windows normally.
-  if (window && CheckForFullscreenWindow()) window->IgnoreXULSizeMode(true);
+  if (nsCOMPtr<nsIBaseWindow> baseWin = do_QueryInterface(aParent)) {
+    nsCOMPtr<nsIWidget> widget;
+    baseWin->GetMainWidget(getter_AddRefs(widget));
+    if (widget && widget->SizeMode() == nsSizeMode_Fullscreen) {
+      window->IgnoreXULSizeMode(true);
+    }
+  }
 #endif
 
   widget::InitData widgetInitData;