Now that I've solved 202 puzzles, wanted to drop by and communicate what an absolute joy it's been to pick my way through TFMoF slowly over the last month or so!
Since you mentioned you'll be working on a similar project in the future, wanted to sneak in a feature request, assuming the limitations of GBstudio empower you to consider it:
When holding down the Fill or X button and holding a direction to quickly mark a row or column, if the starting space was blank, consider not allowing it to overwrite existing fills or crosses on spaces the player has already marked. (this is especially effective when other info has already Filled the required clues in a row or column, as it lets you quickly X off all the remaining blanks without disrupting Fills!)
Thinking deeply about my preference for "efficiency" behaviors like the one above, and given my relative disdain for some of the more advanced "helper" features that modern mobile nonograms tend to have, (like highlighting clues to tell you which ones are being fulfilled) it occurs to me that almost all such requests are matters of preference that might cause a designer to have to implement tricky options menus and toggles; it's impossible to please everyone! So FWIW, I appreciated the old school functionality of TFMoF, (taking me all the way back to Mario's Picross on the Game Boy!) much MORE than I happened to be frustrated by held fills blizting across my previous deductions.
