In 0.85b1 toolbar buttons are not used, it's good, but I still need to press Tab 10 times to go back to the file list, I think it's not needed.
I don't use some panels (Categories / Category Sets), but the focus is still switched to these (closed) panels.
If those panels are displayed, focus is switched also to some fields and buttons (screenshot), I don't think it's needed. Moreover, the order is quite random.
When using keyboard, Tab is useful, but not if I have to press it 10 times, then even mouse is faster.
Perhaps another shortcut could be used to switch between all panels/buttons (Ctrl+Tab / Shift+Ctrl+Tab ?) and Tab (+ Shift+Tab) could be used just to switch between the most important items:
file list > tree view (folders or fav. or categories) > address path > quick search > file list...
MP 0.85 beta 1 64-bit, Windows 10 Home 64-bit.