Summary:
The killer can only kill X11 windows. It is also only invoked for Client
and not for ShellClient. The tool interacts with QX11Info which means it
would crash if tried to use on Wayland.
Thus force platform xcb on the tool.
Test Plan: Killer shows for Xwayland windows.
Reviewers: #kwin, #plasma_on_wayland
Subscribers: plasma-devel, kwin
Tags: #plasma_on_wayland, #kwin
Differential Revision: https://phabricator.kde.org/D3228
f3325a1cb8