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Gray area at the top of fullscreen X11 windows

Sometimes fullscreen windows appear to have a gray area at the top where the decorations would be if the window was not fullscreen. This is due to _MUTTER_FRAME_EXTENTS not getting updated, which causes the client window to have an offset compared to the frame window. The frame window is not displaying any header, but the gray window background is still visible. This issue seems very much timing dependent and I have no fully reliable way to reproduce this.

I tried printing the allocated size and the widget position in meta_frame_content_size_allocate() with the following results:

Working:

Aug 01 23:40:23 t460 gnome-shell[1628845]: size alloc: (1x1) 0 37
Aug 01 23:40:23 t460 gnome-shell[1628845]: size alloc: (1920x1080) 0 37
Aug 01 23:40:23 t460 gnome-shell[1628845]: size alloc: (1948x1109) 0 0
Aug 01 23:40:23 t460 gnome-shell[1628845]: size alloc: (1920x1080) 0 0

Another working case:

Aug 01 23:40:44 t460 gnome-shell[1628845]: size alloc: (1920x1080) 0 37
Aug 01 23:40:44 t460 gnome-shell[1628845]: size alloc: (1948x1109) 0 0
Aug 01 23:40:44 t460 gnome-shell[1628845]: size alloc: (1920x1080) 0 0

Bug:

Aug 01 23:40:49 t460 gnome-shell[1628845]: size alloc: (1x1) 0 37
Aug 01 23:40:49 t460 gnome-shell[1628845]: size alloc: (1920x1080) 0 37

Another buggy case:

Aug 02 00:27:28 t460 gnome-shell[1628845]: size alloc: (1920 1080) 0 37

So it looks like the size of the MetaFrameContent is allocated while the headerbar is still affecting its position (so point.y == 37). The size allocated here is already the fullscreen size, so there will be no further calls to meta_frame_content_size_allocate() and _MUTTER_FRAME_EXTENTS stays at 0,0,37,0 even when the window is in fullscreen mode and there is no actual headerbar to be shown.

xwininfo of the client window 
xwininfo: Window id: 0x800014 "Absolute Drift"

  Absolute upper-left X:  0
  Absolute upper-left Y:  37
  Relative upper-left X:  0
  Relative upper-left Y:  37
  Width: 1920
  Height: 1043
  Depth: 24
  Visual: 0x3c
  Visual Class: DirectColor
  Border width: 0
  Class: InputOutput
  Colormap: 0x800010 (installed)
  Bit Gravity State: ForgetGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +0+37  -0+37  -0-0  +0-0
  -geometry 1920x1043+0+0
xwininfo of the frame window 
xwininfo: Window id: 0x400117 "Absolute Drift"

  Absolute upper-left X:  0
  Absolute upper-left Y:  0
  Relative upper-left X:  0
  Relative upper-left Y:  0
  Width: 1920
  Height: 1080
  Depth: 32
  Visual: 0x354
  Visual Class: TrueColor
  Border width: 0
  Class: InputOutput
  Colormap: 0x400001 (not installed)
  Bit Gravity State: NorthWestGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +0+0  -0+0  -0-0  +0-0
  -geometry 1920x1080+0+0
Edited by Sebastian Keller