Tab is hardcoded to "Show Canvas Only", so it is impossible to use it.
It's true for any docker.
It's the way krita works for now. If we redesign the focus system, we might get it to work, but as long as tab is a global shortcut, it'll work this way. We might start working around -- maybe adding an event filter to every docker that grabs all global key-presses and filters them for some, but that would be really complicated. Note that Tab isn't hard-coded, it's a shortcut that's editable for the user.