Thank's, Quasar, but I don't want dock it.
When the midi editor is undocked, I want sized it to fit my whole screen with a shorcut action.
Not possible, I think
I don't think there is an 'action' to maximize the MIDI editor, but the forum folks let me know you can double-click on the title bar to maximize it (2nd dbl-click restores to previous state). (This method of maximizing a window seems to be a Windows convention - but I'd never used it before).