How do I solve this brush error?

[McGrinyTubbie]

I draw something and I make sure it doesn't have gaps and the paint leaves the selected area
Please help it is very annoying

[behreajj]

Check that the contour not only has no gaps, but that all of the contour is on the same layer.

When the paint bucket tool is active, look at the context bar above the canvas. Check that the tolerance is 0 and that contiguous is ticked.

Depending on the thickness and shape of the contour, neighbor connectivity may be an issue. Click on the button to the right of the contiguous to open the paint tool options menu. If 8-connected is active, try switching to 4-connected.

In the same menu, refer to active layer vs. visible layers will change how the paint bucket behaves when the contour is split across different layers.

If symmetry is turned on for either the x-axis or y-axis or both, then turn it off.

If you want to see some pictures, the section on paint bucket can be found in the menu on the right hand side:
https://steamcommunity.com/sharedfiles/filedetails/?id=3141877152