[Wayland-bugs] [Bug 727452] 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

--- Comment #16 from Simon McVittie <simon.mcvittie at collabora.co.uk> 2014-04-04 12:32:51 UTC ---
(In reply to comment #15)
> (In reply to comment #14)
> > The comments, and the implementation in GdkWindow, suggest that it's meant to
> > be OK for it to have undefined contents - so the patch from Comment #8 would be
> > a more correct approach, if it worked (unfortunately it doesn't, and I still
> > can't see why not).
> >
> I think this is the approach we should take at least in the stable branch.
> 
> But why doesn't this work? Your comment above says you don't use it because you
> feared it'd be slow.

Sorry, I might have been unclear. I've put four patches on this bug:

Attachment #273425 ("#if 0" the fast-path to make it work like the X11 backend)
works, but I worried that it would hurt performance by adding a copy into the
most common case. On the other hand, it sounds as though in practice there's
always a copy on Mutter-Wayland anyway. I think this is the only one so far
that both works, and is theoretically correct.

Attachment #273427 doesn't actually seem to prevent the symptom for me, and I
haven't been able to work out why not.

The pseudo-patch in the text of Comment #8 is a more refined version of
Attachment #273427 in response to Daniel's comments, but also doesn't seem to
work. Again, I haven't been able to work out why it doesn't do what I think it
should.

Attachment #273458 prevented the symptom when I tried it, but you pointed out
that it was wrong for the cases where we only redraw parts of the window, and I
agree.

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