Unfortunately, no there is no visual editing in the default editor. I'm just waiting for the modified version
First, what you are putting down: It automatically uses the tileset to fill the box in. in the center, the tiles are randomly chosen from the different fill blocks at runtime. (so every time you play that room, it will be different)
Delimit mode:
All: If you have two blocks overlapping, it will treat it as one block, and only do the outline tiles around the edge of the merged shape
Same: It will only merge blocks of the same tiles.
None: It will not merge to any other blocks