Areas in NSMBDS have a dimension of 32x16 chunks, or 512x256 tiles. The coordinates start at 0, meaning the max X coordinate is 511, and the max Y coordinate is 255. However, for some reason, the editor won't scroll past X coordinate 509 and Y coordinate 253, meaning there are two inaccessible tile rows and columns. Overall, a very minor issue that most people haven't noticed, but an issue nonetheless. While I don't hold much hope for this to be fixed considering how old the last official build is, I figured it wouldn't hurt to have these issues documented in case someone wants to tackle them someday.
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