1. Try to save (Copy/Copy/TS) to a read-only file.
1r. Asked to overwrite, then "Info > Muxer > Cannot open".
The message looks like the muxer itself could not be opened.
Would prefer a dynamic message like "Error > File Save > File xyz is read-only."
(Not sure whether that should even happen before/instead asking to overwrite.)
Should check what other applications do...