There seems to be a few things that elude me regarding shortcuts. Precisely, I'd like to have Ctrl+Shift+Tab take me to the previous partition tab, the same way that Ctrl+Tab takes me to the next tab.
My use case is that I switch back and forth to visually look for differences in realizations of exercises. I'm already used to toggling between views, in other apps, using Ctrl+Tab - Shift+Ctrl+Tab pairs.
I went to Edit > Preferences… > Shortcuts, looked for Previous score , which lists 3 shortcuts registered (Ctrl+Shift+Backspace; Ctrl+Shift+F6; Back), but couldn't define Ctrl+Shift+Tab as a new one, because that key combination doesn't trigger the input field (it needs a “not dead” key, like a letter or digit; plus, as one would expect, Tab moves the focus to the next control inside the Define… pop up).
Delving a bit deeper, I tried to look for something like a .ini file to edit, where I could add that shortcut. I found %User%\AppData\Local\MuseScore\MuseScore3\shortcuts.xml (and thought to myself “an .ini file… am I so old already?!” X-D ). I replaced:
<SC> <key>previous-score</key> <std>21</std> </SC>
<SC> <key>previous-score</key> <std>21</std> <seq>Ctrl+Shift+TAB</seq> </SC>
Upon re-launching MuseScore, my preferred sequence is now available, but the 3 default ones (listed at the beginning of this post) aren't any more. This situation is ok for me as it is right now. However, the documentation  lists Ctrl+Shift+Tab as the shortcut for the previous score action, which makes things a tiny bit confusing — I've heard about the recent major updates in the app, some discontinuity is indeed to be expected ;) So, while I don't have a question or support request any more, it might be desirable to take some action; I just don't know enough about the project development to guess as to what change(s) that might be, or what kind of ticket to post about it. Thanks for bearing with me any way ;)