Normally you can use the terrain tool to add terrain and if you mess it up or want to change what you painted, you could right click on an empty terrain area and it will delete the terrain instead. This option no longer works on the newest patch. Was it on purpose? If so, I really enjoyed that feature.
Sorry, this appears to be a regression in 1.1.4 vs. 1.1.3 caused by the following fix:
The annoying part here is that while in general we want to consider a “checked” cell equal to an “unchecked” cell, they really should be considered different in TileLayer::setCell, but due to the above change they aren’t. This is breaking the erasing mode of the Terrain Tool.
I pushed the following fix for now, which will be included in Tiled 1.1.5 and the next development snapshot: