Bug: using the Fill tool while holding Alt fails to replace turfs

[mcbalaam]

Version

v2.15.0.alpha

What happened?

While replacing borders of my hotel room with space turfs I suddenly realized that using the Fill tool while holding Alt (to replace turfs, as the description states) fails to replace turfs, leading to closed and/or open turfs stacking:

Reproduction

1. Run StrongDMM
2. Open any map
3. Pick any turf, open or closed
4. Switch to the "Fill" tool and fill out an area while holding Alt
5. See that turfs are now stacked

Relevant log output