fix(linux/xdgportal): allow unified systemd service for all capture methods (#4854)

docs/troubleshooting.md

@@ -160,18 +160,21 @@ sudo usermod -aG input $USER
 ```
 
 ### KMS Streaming fails
-If screencasting fails with KMS, you may be using the unprivileged app-dev.lizardbyte.app.Sunshine service unit.
-Switch to the privileged sunshine service:
+KMS screencasting requires elevated privileges which are not allowed for Flatpak or AppImage packages.
+This means that you must install Sunshine using the native package format of your distribution, if available.
+KMS capture will soon be phased out in favour of XDG Portal Capture (which works with all package types).
+
+### KMS Streaming; some windows flicker/disappear on KDE Plasma 6.5+
+KWin's overlay support interferes with KMS capture. As of KWin 6.5 this is not yet set by default, but
+for future versions that enables this by default, you may be able to disable again via a special
+[environment variable](https://invent.kde.org/plasma/kwin/-/wikis/Environment-Variables#kwin_use_overlays):
 
 ```bash
-systemctl --user --now disable app-dev.lizardbyte.app.Sunshine
-systemctl --user --now enable sunshine
+export KWIN_USE_OVERLAYS=0
 ```
 
 > [!NOTE]
-> The above commands will not work with the AppImage or Flatpak packages, as KMS screencasting
-> requires elevated privileges which are not allowed by their respective packaging security policies.
-> As an alternative, XDG Portal capture is recommended.
+> Disabling overlays will reduce KWin's rendering efficiency. Consider using XDG Portal Capture instead.
 
 ### KMS streaming fails on Nvidia GPUs
 If KMS screen capture results in a black screen being streamed, you may need to