Do not overwrite existing event mask of root window

QXcbScreen installs its own event mask on the screen's root window.
This overwrites any existing event mask already set and by that
breaks applications when a new screen is added.

By first fetching the existing event mask and adding it to the newly
installed event mask, Qt does no longer break applications also
installing an event mask on the root window.

Task-number: QTBUG-39648
Change-Id: I8686dd6ae49d0e807c6fe1ea4a231ff728bfcf25
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/plugins/platforms/xcb/qxcbscreen.cpp b/src/plugins/platforms/xcb/qxcbscreen.cpp
index 9f19841437..01e78465b6 100644
--- a/src/plugins/platforms/xcb/qxcbscreen.cpp
+++ b/src/plugins/platforms/xcb/qxcbscreen.cpp
@@ -106,6 +106,11 @@ QXcbScreen::QXcbScreen(QXcbConnection *connection, xcb_screen_t *scr,
     qDebug("  root ID........: %x", screen()->root);
 #endif
 
+    QScopedPointer<xcb_get_window_attributes_reply_t, QScopedPointerPodDeleter> rootAttribs(
+        xcb_get_window_attributes_reply(xcb_connection(),
+            xcb_get_window_attributes_unchecked(xcb_connection(), screen()->root), NULL));
+    const quint32 existingEventMask = rootAttribs.isNull() ? 0 : rootAttribs->your_event_mask;
+
     const quint32 mask = XCB_CW_EVENT_MASK;
     const quint32 values[] = {
         // XCB_CW_EVENT_MASK
@@ -113,6 +118,7 @@ QXcbScreen::QXcbScreen(QXcbConnection *connection, xcb_screen_t *scr,
         | XCB_EVENT_MASK_LEAVE_WINDOW
         | XCB_EVENT_MASK_PROPERTY_CHANGE
         | XCB_EVENT_MASK_STRUCTURE_NOTIFY // for the "MANAGER" atom (system tray notification).
+        | existingEventMask // don't overwrite the event mask on the root window
     };
 
     xcb_change_window_attributes(xcb_connection(), screen()->root, mask, values);