Qt 5.9 and older KWin

[Martin Flöser]

Dear distributions,

unfortunately I have to inform you that Qt 5.9 includes a change [1] 
which causes a severe regression in KWin/X11 (Wayland is not affected). 
The regression results in keyboard input not working in effects like 
Present Windows. It affects any KWin version.

We have created a workaround for Plasma 5.10.3 [2]. In case you are 
already shipping Qt 5.9 together with Plasma 5.10 I advise to 
cherry-pick this commit.

Please note that the change will not be backported to neither Plasma 5.9 
(as there are no more releases planed) nor to Plasma 5.8 LTS. The change 
does not fit the nature of an LTS release as it is a workaround for a 
future software. I highly advise against updating Qt on systems running 
Plasma LTS. Please note that I consider it as likely that we have other 
areas in Plasma affected by the Qt change and we just haven't identified 
them. If you combine Plasma 5.8/5.9 with Qt 5.9 I advise to cherry-pick 
the commit.

Best Regards
Martin Flöser
KWin maintainer

[1] 
https://code.qt.io/cgit/qt/qtbase.git/commit/?h=5.9&id=2b34aefcf02f09253473b096eb4faffd3e62b5f4
[2] a6dee74ee455d1da47dd5c9d55a84adbb5e1426a