Moving map sometimes doesn't repaint correctly

(Moving https://sourceforge.net/apps/mantisbt/tiled/view.php?id=84 to the new issue tracker here)

When the map is being dragged around by either middleclick or by space bar and left mouseclick, sometimes
artifacts (the current stampbrush) is painted on the map and not removed correctly.

That happens when you scroll against a border (so mouse moves, but map does not move, because map border is reached)
and once you are done scrolling, the area is not repainted until you move the mouse.

Reproduce the issue in the map editor by dragging with middle-click or space plus left-click into a map border, then stop scrolling. Trace the repaint path for the stampbrush artifact when the map cannot move; done means the affected area repaints immediately without requiring another mouse movement.

Written by the indexing model from the issue text.