May be I do not understand the problem right. But the described behavior sounds correct to me. Flood fill should stop on another (not the label you clicked on (including background) label boundary. The only thing you could discuss is wether it is important if another label is locked or not.

In T28763#229375, @floca wrote:

May be I do not understand the problem right. But the described behavior sounds correct to me. Flood fill should stop on another (not the label you clicked on (including background) label boundary. The only thing you could discuss is wether it is important if another label is locked or not.

Maybe a quick screenshot-example shows what I find disturbing:
Click with "Fill" tool somewhere outside of the red label results in:

Click with "Fill" tool inside the hole of the red label results in:

In the second case I was expecting the blue label to be filling the whole image, leaving out the area of the red label (no overlap), like it is shown in the first screenshot.
Because when the blue label is calculated, it looks for the boundaries of the blue label. There are no boundaries so the whole image is filled - however, the red area is not "painted over".
If this is not the expected result, because the boundaries of the red label are taken into account (as it is done in the second screenshot), then I guess the first case is wrong.

Btw., I also tested it with unlocked labels and here the behavior of both cases is also different: Filling outside the area creates a blue label on the whole image, painting over the red label.
Filling inside the hole only fills the hole, no "painting over".