[Wayland-bugs] [Bug 727452] New: Wayland windows with non-trivial alpha get new content drawn over old

[gtk+ (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=727452
  gtk+ | Backend: Wayland | 3.12.x

           Summary: Wayland windows with non-trivial alpha get new content
                    drawn over old
    Classification: Platform
           Product: gtk+
           Version: 3.12.x
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: Backend: Wayland
        AssignedTo: gtk-bugs at gtk.org
        ReportedBy: simon.mcvittie at collabora.co.uk
         QAContact: gtk-bugs at gtk.org
                CC: rob at robster.org.uk, marco at barisione.org,
                    wayland-bugs at lists.freedesktop.org
     GNOME version: ---


GtkWindowImplWayland tries to optimize by doing the Gtk drawing directly
into the same buffer it gives to Wayland, as long as Wayland is not
using that buffer right now.

Normally, this would be fine, because we clear that buffer to the
window's background pattern with gdk_window_clear_backing_region()
before painting it.

However, if you're making a fancy composited shell with semitransparent
bits, the background pattern might itself have an alpha channel. If it
does, at the moment we just paint it over the top of whatever happens
to be left in the buffer from the previous frame, which will shine
through any areas with non-trivial alpha.

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