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Steps to Repro:
1 - Open a new map.
2 - Select a tile.
3 - Click (do not release)
4 - Drag around inside the maps edge.
5 - Observe tiles are continuously drawn.
6 - Without releasing the mouse button - drag the cursor outside the map area (into the grey).
7 - Drag it back inside the map's edge.
8 - Observe tiles are no longer drawn.
This is probably pretty low priority - but it doesn't behave as I might expect it should.