Are you really using Gimp 2.10.4 - that is very old, the current is 2.10.28
However, difficult to say without you actual layers set-up. Do you have a layer with an extended boundary ? Can come from cropping in image.
Maybe a change in work flow (or adapt to your circumstances)
Make the bottom layer active
Image -> Fit canvas to layers (to remove any extended boundary)
Toggle visibility off for all except base layer. If you have lots of layers, the shift key will toggle all on/off
Image -> Canvas Size with
Set increased height
Resize - All visible layers
Fill with: Foreground Colour (set to black)
Toggle layer visibility back on.
40 second demo of that https://i.imgur.com/GgWERhC.mp4
Post a screenshot of the whole Gimp interface for a better answer.
However, difficult to say without you actual layers set-up. Do you have a layer with an extended boundary ? Can come from cropping in image.
Maybe a change in work flow (or adapt to your circumstances)
Make the bottom layer active
Image -> Fit canvas to layers (to remove any extended boundary)
Toggle visibility off for all except base layer. If you have lots of layers, the shift key will toggle all on/off
Image -> Canvas Size with
Set increased height
Resize - All visible layers
Fill with: Foreground Colour (set to black)
Toggle layer visibility back on.
40 second demo of that https://i.imgur.com/GgWERhC.mp4
Post a screenshot of the whole Gimp interface for a better answer.