I know, it has been almost 8 yrs since this was posted but since I ended up here looking for the solution I thought I would say that Gyzzorn was correct. Ordering the buttons on the left panel, top-to-bottom = direction of tabbing. This works perfectly in 5.2.1 and it should be noted that if a UI element (text box, combo, etc) is disabled during gameplay then it is simply skipped during the user’s tabbing process.