I still think you are scaling a one-pixel high selection. Have you up-graded to Big Sur where the selection is there but does not show ? Which version of Gimp ? It looks like Gimp 2.10.14 or there-a-bouts. Not paying attention Gimp 2.8 given.
Did you try Select -> None. Then apply the scale tool.
Your screenshot is a bit fuzzy but this is a what I think is close. This in linux / Gimp 2.10.14 https://i.imgur.com/i7TywBG.mp4 I hope you can play one of these in MacOS.
Did you try Select -> None. Then apply the scale tool.
Your screenshot is a bit fuzzy but this is a what I think is close. This in linux / Gimp 2.10.14 https://i.imgur.com/i7TywBG.mp4 I hope you can play one of these in MacOS.