I noticed that at one time, I think, at least, you had implemented something in the map editor which acts differently for a right-click than a left-click. I can't find the post where I said something about this, so I'll create this one. Also, in the latest alpha (4) I can't seem to find this functionality, so now I wonder if I'm just going crazy. In spite of that, here's this:
What I was suggesting was when you right-click on a tile on the map, it selects that tile to be the current tile to be placed by left-clicking. What it looks like happened is that you can simply have two selected tiles to place, one with LMB, one with RMB. While this has some use, I think the other option would be much more useful (I know I would find it much more useful, anyway). If, for example, you accidentally draw over one tile too far into another block of tiles, rather than finding the tile you want to put back in the list on the left, you can just copy it from the other tiles that are correct and put it right back where it belongs.