React to Shift+Tab in Gloss

Question

I am currently writing a game using gloss' play. The basic structure of this game is very simple: I have a board with units on it, one of which is in focus.

I have added the ability to change the focus by cycling through the units on the board using KeyTab. Quite quickly, I realised that I wanted to also be able to cycle back so I thought I'd use Shift + KeyTab to do so as is customary in this situation.

gloss' Events come with Modifiers, one of which is shift. Perfect? Well... not quite: whenever I press Shift + KeyTab, the event is never received by the window (I used Debug.Trace's trace to print all the received Events).

With CAPS LOCK, I do receive KeyTab with the shift modifier marked as Down. If I switch to using Ctrl + KeyTab as my shortcut, it works perfectly well. Here is a minimal example reproducing the problem:

module Main where

import Graphics.Gloss
import Graphics.Gloss.Interface.Pure.Game

data Board = Board Color Color Color

forward :: Board -> Board
forward (Board x y z) = Board y z x

backward :: Board -> Board
backward (Board x y z) = Board z x y

displayPoint :: Point -> Color -> Picture
displayPoint (x, y) c = translate x y $ color c $ circle 10

displayBoard :: Board -> Picture
displayBoard (Board x y z) =
  Pictures $ zipWith displayPoint coordinates [x, y, z]
  where coordinates = [(0, 30), (-30, -30), (30, -30)]

react :: Event -> Board -> Board
react (EventKey (SpecialKey KeyTab) Down mods _) b =
  case shift mods of
    Up   -> forward b
    Down -> backward b
react _ b = b

main :: IO ()
main =
  let window = InWindow "Buggy" (100, 100) (200, 200)
      board  = Board blue red yellow
  in play window black 24 board displayBoard react (const id)