Problem with Warning on Immobilizing Terrain

[RightDeve]

If a vehicle wants to enter a rough-slope/building hex, while there's already a friendly unit on that hex e.g infantry, it must use the Shift key to do so. But pressing the Shift key and clicking on that hex will only show the warning ("Enter yes/no?") very briefly before it vanishes. No time to input anything. Sometimes it will show up, but only if the button & click are pressed with accurate timing.

I have only just encountered this while playing Scenario 17: South of Beirut.