Summary:
When a window is being interactively resized, its contents may jump. The
reason why that happens is because KWin renders partially resized client
window. Composite extension spec says that a window will get a new pixmap
each time it is resized or mapped. This applies to the frame window, but
not to the client window itself. If the client window is resized,
off-screen storage for the frame window won't be reallocated. Therefore,
KWin may render partially resized client window if the client doesn't
attempt to be in sync with our rendering loop. Currently, the only way
to do that is to use extended frame counters, which are not supported by
KWin.

So, in order to fix visual artifacts during interactive resize, we need
somehow forcefully re-allocate off-screen storage for the frame window.
Unfortunately, Composite extension doesn't provide any request to do
that, so the only option we have is to resize the frame window.

BUG: 415839
FIXED-IN: 5.18.0

Reviewers: #kwin

Subscribers: davidedmundson, ngraham, alexde, fredrik, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26914