it does fill the entire mesh of the selected color/mask, or the inner of a designed area within the same painted layer
Justification
useful to easily erase the paint, like the clean button for the paint node in the canvas, useful also for filling zones ( e.g. you paint a polygon silhouette and instead to hand fill the inner, you can just click in it’s inner and it will fill it automatically.
Implementation Details
it can be an option of the brush like the symmetry or it can also be a separated tool like the paint projector