In-game, such windows are drawn as normal, simply with directional indicators displayed at the start of the level and in Clear Physics Mode.
An indicator would need to be added if the draw-flipped is fixed, so I haven't done that yet. I have implemented a semi-fix in commit 3177040, which ignores flipping for the purpose of drawing the window's trigger area. Not perfect, but it's good enough to at least give an accurate portrayal of the physics of the window.
Why this matters: Yes, all (or most?) current windows are close to symmetrical, but (a) many aren't perfectly symmetrical, and (b) this doesn't mean all future ones will be. If we're only worried about (a), my semi-fix should be good enough alone.