How can I leave a selection as is when I change layer?

[makois]

I'm a beginner at GIMP living in Japan  

Normally, If you make a selection then change the current layer, the selection remains.
However, If you make a selection of the whole layer(Ctrl+A)(A) then change the current layer(A to B), the selection changes to the changed layer's whole size(B).
How can I leave the selection size as is?
( Please see the attached image. )

Environment:

• Gimp 2.10.14
  
• macOS Catalina
  

[Blighty]

Use the attached script. Put it in the scripts folder of your user profile.

You will find it in Select > Layer to selection

[Ofnuts]

A selection is an image-wide object. Select>All is really selecting all pixels in the image. It looks like it is a selection on the active layer because the actions are always on the active layer intersected with the selection (the marching ants are showing this, but where the selection is outside the active layer, the ants are still).

[rich2005]

Using Basic Gimp.

Select All does just that - selects the bounds of the whole image.
To select just the extents of the small layer, Two ways depending on the layer.
For a completely opaque layer Use Layer -> Transparency -> Alpha to selection
or
More general, Use the Fuzzy select tool, Set the threshold to maximum (255) and click in the layer.

Quick explanation / example using alpha-to-selection method 1 minute 30 seconds https://youtu.be/tZVE1YPDfJI

[makois]

Thanks all!
I have learned Select -> All makes a selection of the image.
I appreciate the information and the method that I wanted and the helpful script.