T21: on X11 the window names its pane in _KDE_NET_WM_BLUR_BEHIND_REGION and drops it when Blurred goes; Ely's new ResizeBorder::glass leaves a client-drawn window's pane to the content, without ground or shadow, so a Linux window shows the desktop at all; T20's region and this property cover the pane alone, worked out each frame and sent only on a change; the Ely pin moves to 5dc4dd1; the X11 test runs its own Xvfb

.github/workflows/ci.yml

@@ -35,7 +35,7 @@ jobs:
       # The Linux job container runs as root, which ignores the file permissions the tests check.
       - if: runner.os == 'Linux'
         run: |
-          apt-get update && apt-get install -y zsh fish
+          apt-get update && apt-get install -y zsh fish xvfb
           useradd --create-home ci
           chmod o+x "$HOME"
           chown -R ci "$GITHUB_WORKSPACE" "$HOME/.cargo" "$HOME/.rustup"