Changing users shortcut muscle memory... But adding additional toggle keybindings might be more acceptable.
Since the title of the issue does say "add", it was probably ralf3u's intention that this would be an additional shortcut. That is how I would have implemented anyway.
I was thinking the lua plugin could be used for something like this, but I didn't see any functions for checking visibility and grabbing focus in the docs.
I usually want to go to a specific tab...
That's true. I normally use the mouse.
Responses for maintained plugins come from plugin maintainers, Geany hasn't heard from the markdown maintainer for a while, maybe he is busy, did you ping him?
I pinged codebrainz in the issue last week. I'm not in a hurry and figured he might be busy. Since I fixed the issue for the preview plugin, I thought I might as well send the fix on to the markdown plugin.