In most GUIs pressing tab will take you to the next UI control. Simutrans implements this. Pressing shift+tab will take you to the previous one - which Simutrans doesn't implement.
I think this would be a good addition for the UI :)
The code seems to already be there for this, investigating...
Yes, this works at present here (svn 5547, Linux)...
Hm, ok it must be something peculiar to the Mac platform then, doesn't work either using my Quartz backend or the SDL one.
Does shift work elsewhere in the game?
Yep!
I found the problem, on the Mac pressing Shift+Tab sends a special keystroke, NSBackTabCharacter (0x0019). I've fixed this for the Quartz backend by converting this into a SIM_KEY_TAB with the shift modifier set.