"Godot 4 seems fun, I should get used to it with a simple project. Picross is a simple game, why don't I try that?"
↑ clueless
GridContainers and Buttons - a match shockingly made in heaven. Of course, buttons probably aren't the best choice here. It'll be changed to Panels at... well, some point. Either way, I've gone into this project thinking it would be simple and easy. And here I am, two days later, celebrating the victory of the column counter properly processing having 0 tiles. Needless to say, counting the number of tiles in a grid is easy... figuring out what row and column they're in is not.
And I haven't even gotten to the difficult part yet! I still have yet to whip up some code that can detect traditional Picross counting - like having 2 2 for a row that has four tiles with two of them being separated by an empty tile. And then there'll be solving, and clicking and dragging, and marking, and crossing... I underestimated puzzle devs. And for that, I apologize.