getGeoDecorations

Open polylines drawn inside the outline (the X of an x-box, the tick of a check-box).

strokeWidth shortens the marks that end *on* the outline. A stroke is centred on its path and its round cap reaches half a width beyond the last point, so diagonals running corner to corner of the box came out as four spikes sticking through the very outline they belong inside. Pulling each end back by half a width lands the cap on the corner instead of past it. The check-box's tick sits well clear of the edge and is left alone.