Thank you for your help.
I'm trying to snap the frame border to the very edge of the image, but I'm having trouble.

[Sample image]
2400 x 3200px, 72dpi

[Frame border]
Thickness: 30px

[What I want to do]
I want the frame border to snap to the very edge of the image.
(So that the entire 30px width is drawn when output.)

For example, when I press the ▼ button at the bottom of the frame border to expand it,
Ideally, the frame line should stop at the very edge of the image, just where it would appear within the image.

Also, it would be helpful if it could snap to a similar position when I manually move the frame border.

Currently, when I press the ▼ button, the frame line goes past the edge of the image and stops just outside the image.

Even when I manually adjust it to follow the edge, there are errors, such as it going a few pixels over or going inside.

I haven't set the basic frame settings for the manga manuscript, so will it work if I set this?
Other methods are fine too, so I'd appreciate any advice.