The plain-Linux overlay re-enable (#53185) left nativeOverlayWidth() at 0
for plain Linux, so the native min/max/close buttons painted on top of the
app's right-edge titlebar tools. Reserve the fallback width everywhere the
WCO overlay is painted (Windows, WSLg, plain Linux); macOS still reserves 0
since it uses traffic lights.
Live-measure WCO width in the renderer, drop the right rail below the titlebar
band, and re-enable GPU compositing under WSLg when /dev/dxg is present.