Inline Fill Tool
This is an idea for a new tool.
I haven't seen it in any other pixelling program ever but it would be a HUGE time-saver for me.
It would work like this:
You click somewhere on the picture and similar to the fill bucket it would affect the single-coloured area around the pixel you clicked on. It would draw a single-pixel line INSIDE this area using the FG colour. So it would work like the opposite of the brush outline function.
You could also use it for quick outlining without picking a brush first by clicking outside the shape to be outlined.
It could be another state for the fill-bucket button.
I am attaching images to illustrate the behaviour.