A hidden overlay window stops producing frames once it is explicitly
hidden and then reloaded (the Yomitan content-script reload right after
startup), so ready-to-show never fires and the content-ready gate keeps
the window hidden forever behind the Overlay loading spinner. Mark
content ready from did-finish-load after a 1.5s grace period so the
overlay always becomes showable; ready-to-show still wins when it fires.
