xcb: rely on WM_SIZE_HINTS gravity to handle x,y positioning

The original code was added by b316c3ac5e4acac75505bfd77677cecc181599a
(in 2012). This patch changes two things:

1) We now rely on WM to position a window based on the set gravity. It
should not be necessary to calculate coordinates manually as was done
in windowToWmGeometry(). We don't even know the decoration size before
the window is mapped.

2) We now update gravity whenever needed instead of hardcoding based
on what Qt APIs (setGeometry vs setFramePosition) where used to set
the initial window position.

The patch from b316c3a says:

"Determine gravity from initial position. Do not change later as
it will cause the window to move uncontrollably"

Since it did not elaborate on the situation, we can only assume
that it was caused by another bug in Qt at the time or perhaps
a broken WM. From [1]:

"Applications are free to change their win_gravity setting at any
time. If an Application changes its win_gravity then the Window Manager
should adjust the reference point, so that the client window will
not move as the result."

Tested on Ubuntu/Unity, KDE/KWin, Gnome-shell/Mutter, Lubuntu/OpenBox.
Works as expected everywhere expect Unity. Unity seems to ignore
XCB_GRAVITY_STATIC and treats it as XCB_GRAVITY_NORTH_WEST, which means
that setGeometry/setFramePosition produce the same placement on this WM
(the behavior was the same also before this patch).

P.S. Also renamed xRect -> rect, which was a leftover from
ae5f2a66720a4bb22c120bd7d1564652cac00367

With this change we can un-blacklist QWidget save/restore geometry
auto tests.

[1] https://specifications.freedesktop.org/wm-spec/latest/ar01s09.html

Task-number: QTBUG-66708
Change-Id: I381eef5d34dddb04de16a897ce5540b9c430b216
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

1 files changed, 0 insertions, 2 deletions

diff --git a/src/plugins/platforms/xcb/qxcbwindow.h b/src/plugins/platforms/xcb/qxcbwindow.h
index b44ba77a6b..d75f22bb51 100644
--- a/src/plugins/platforms/xcb/qxcbwindow.h
+++ b/src/plugins/platforms/xcb/qxcbwindow.h
@@ -253,8 +253,6 @@ protected:
 
     Qt::WindowStates m_windowState = Qt::WindowNoState;
 
-    xcb_gravity_t m_gravity = XCB_GRAVITY_STATIC;
-
     bool m_mapped = false;
     bool m_transparent = false;
     bool m_usingSyncProtocol = false;