Descriptive Problem Summary: When deleting a portion of the map using the Select Rectangle tool, followed by adding a turf with partial coverage, world.turf is deleted.

Numbered Steps to Reproduce Problem:
1) Launch a project with a map and world.turf set to a /turf with an icon.
2) Ensure that the map editor has been set to Insert mode for turfs.
3) Using the Select Rectangle tool, delete a portion of the map. Observe that world.turf has taken its place.
4) From the palette, select a /turf that has an icon that does not cover the entire tile.
5) Add the /turf selected in step 4 to the portion of the map that was deleted in Step 3.
6) Observe that the world.turf has been replaced with the /turf selected in step 4.

Expected Results: For the world.turf to be added as an underlay to the newly placed turf.

Actual Results: The newly placed turf replaces world.turf in the area deleted by the Select Rectangle tool.

Does the problem occur:
Every time? Or how often? Every time.

When does the problem NOT occur? This does not occur with the Eraser tool.