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I was wondering if you can make a tool that can detects gaps in your drawing? It's not an ON/OFF Switch, it's more if you press and hold the key the gaps detection starts if you release the key it immediately stops the gap detection and you can hop on and off it quickly. I prefer a Full Screen Gap Detection but maybe it'll hit CPU a bit? So I was thinking if we also adding options to the gap detection, a check option to the gap detection tool, 'the Gap Detection Radius' an option is center on your mouse cursor when you hover, and you can tweak the setting of the 'Gap Detection Radius Size' to conserve CPU. So there's Full Screen Gap Detection for Fast CPU and also Partial Gap Detection for Slow CPU.

hmmm there is nothing like this at the moment.
But I keep the idea in mind.

an idea upon reading this:
Couldn't you make a script that fills with a gap close and expand "on", and then automatically after that, fills again but now with erase and without the expand so that the result is a line that lies under the drawing AND closes the gaps. when you use a bright contrasting color it can function as a gap marker.

What about using a CTG layer (much better than the fill tool at detecting gaps) with a border FX on it and applied to another layer? Using kind of the method described by peter above.