xcb: Use a placeholder QScreen when there are no outputs connected

If no screens are available, windows could disappear, could stop rendering
graphics, or the application could crash.  This is a real use case in several
scenarios: with x11vnc, when all monitors are physically disconnected from a
desktop machine, or in some cases even when the monitor sleeps.  Now when the
last screen is disconnected, it is transformed into a fake screen.  When a
physical screen appears, the fake QScreen is transformed into a representation
of the physical screen.  Every virtual desktop has its own fake screen, and
primary screens must belong to the primary virtual desktop. It fixes updating
screen geometry on temporarily disabled screens in the middle of the mode
switch.

Expected results: Windows don't disappear, the application doesn't
crash, and QMenu is displayed on the appropriate screen.

This reverts patch 51ada7734ad780178ecced11e0dff454dfc2e5f2

Change-Id: I6e8eb682b0c8425d08ffdaecbd4c6c7700c914b4
Task-number: QTBUG-42985
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/gui/kernel/qscreen.cpp b/src/gui/kernel/qscreen.cpp
index fb322ae74f..52e7686439 100644
--- a/src/gui/kernel/qscreen.cpp
+++ b/src/gui/kernel/qscreen.cpp
@@ -116,8 +116,8 @@ QScreen::~QScreen()
     bool movingFromVirtualSibling = primaryScreen && primaryScreen->handle()->virtualSiblings().contains(handle());
 
     // Move any leftover windows to the primary screen
-    foreach (QWindow *window, QGuiApplication::topLevelWindows()) {
-        if (window->screen() != this)
+    foreach (QWindow *window, QGuiApplication::allWindows()) {
+        if (!window->isTopLevel() || window->screen() != this)
             continue;
 
         const bool wasVisible = window->isVisible();