How to stretch the edge of a picture.

[Blighty]

1. Image>Canvas Size and increase size by 15 pixels
https://imgur.com/a/kVPoTOK

2. Right click on layer and Layer to Image Size

3. Make a selection on the left edge of the image
https://imgur.com/a/EyrqtUr

4. Use the Scale Tool to stretch the selection to the left.

5. The scale tool creates a new Floating Selection in the layers dialogue
https://imgur.com/a/smqWgWO

6. Right click the floating selection and select To New Layer

7. (If necessary - use the Move Tool to move the new layer 1 pixel to prevent a gap between the two layers)

8. (If necessary - use Layer Masks to blend the two layers)