Fix AttackMove not working properly under certain conditions

This fixes the issue where AttackMove (and possibly other order modes) would not work properly when people were clicking their mouse buttons at the same time. A move order (the default order mode) would be issued instead.

OpenRA.Game/Widgets/WorldInteractionControllerWidget.cs

@@ -120,12 +120,25 @@ namespace OpenRA.Widgets
 							}
 						}
 					}
-					else if (dragStart.HasValue)
+					else
 					{
-						// Select actors in the dragbox
-						var newSelection = SelectActorsInBoxWithDeadzone(World, dragStart.Value, xy);
-						World.Selection.Combine(World, newSelection, mi.Modifiers.HasModifier(Modifiers.Shift), dragStart == xy);
+						/* The block below does three things:
+						// 1. Allows actor selection using a selection box regardless of input mode.
+						// 2. Allows actor deselection with a single click in the default input mode (UnitOrderGenerator).
+						// 3. Prevents units from getting deselected when exiting input modes (eg. AttackMove or Guard).
+						//
+						// We cannot check for UnitOrderGenerator here since it's the default order generator that gets activated in
+						// World.CancelInputMode. If we did check it, actor de-selection would not be possible by just clicking somewhere,
+						// only by dragging an empty selection box.
+						*/
+						if (dragStart.HasValue && (!(World.OrderGenerator is GenericSelectTarget) || hasBox))
+						{
+							var newSelection = SelectActorsInBoxWithDeadzone(World, dragStart.Value, xy);
+							World.Selection.Combine(World, newSelection, mi.Modifiers.HasModifier(Modifiers.Shift), dragStart == xy);
+						}
 					}
+
+					World.CancelInputMode();
 				}
 
 				dragStart = dragEnd = null;