Local topspace-mode affects other buffers, too

[DivineDominion]

Describe the bug

M-x topspace-mode affects all buffers, not just the local one.

To Reproduce
Steps to reproduce the behavior:

Example:

1. Open Emacs emacs -Q, install topspace
2. Visit file1.el with (require 'topspace) in it.
3. Open other window C-x 3, visiting another file there, file2.el (doesn't have to exist)
4. Run M-x topspace-mode in one of the splits
5. scroll both window splits with the mouse

Expected behavior
The window and its buffer where you ran topspace-mode scrolls down; the other doesn't

Actual behavior
Both windows can scroll down with topspace

Screenshots

Edit: I now realize this variable is not buffer-local by default anyway, and also t after the require, so that's irrelevant. Screenshots left here to show that the mode is off in one of the buffers.

The window where topspace-mode is run:

The window that shouldn't have been affected still has topspace-active set to t:

Emacs version

GNU Emacs 28.0.90 (build 1, aarch64-apple-darwin21.2.0, NS appkit-2113.20 Version 12.1 (Build 21C52))
of 2021-12-28

topspace.el from master

[trevorpogue]

Thanks for catching this. For anyone wondering, the issue was related to the fact that advice-add is used to surround scrolling functions with custom topspace functions, and advice-add naturally is a global operation so even if one buffer had topspace-mode enabled and added advice-add then all other buffers would still call the custom topspace functions even if topspace wasn't enabled in those buffers.

I added some flags in the advice-add functions to guard from this. Also, I clarified that topspace shouldn't be enabled when topspace-mode is nil, even if topspace-active is non-nil.