#1597 Scrolling slowly with external mouse moves document in wrong direction

[Christopher Rooney]

Hello!

Summary Scrolling slowly with external mouse can result in the document moving in the opposite of the expected direction.

Detailed Description

Overview
When using an external mouse, if I scroll downwards slowly (pulling the scroll wheel towards me at fewer than a few clicks per second) the document will actually scroll upwards. If I scroll downwards quickly, the document starts moving in the opposite direction.

Notably, this only affects Skim. If I go to some other app, e.g. Firefox or Safari, scrolling works as I expect. That rules out a mouse malfunction (Also I have noticed this behavior on two different mice I've tried on two different systems)

Steps to reproduce
Ugh, this isn't reproducible. I'm going to post anyway because this might still be a useful workaround?

1. Disable Natural Scrolling for the mouse in system settings.
2. Open a document in Skim. (perhaps a fresh install of Skim or update from a previous version is needed? The first thing I do when setting up a new Mac is disable natural scrolling.)
3. Use the external mouse to scroll downwards very slowly.
4. The document will scroll upwards (expected: document will scroll downwards)
5. Use the external mouse to scroll downwards very quickly
6. The document will scroll downwards now, as expected
7. Enable Natural Scrolling for the mouse in system settings.
8. The same behavior will manifest in opposite directions
9. Reboot Skim (Quit and re-launch)
10. The document will scroll as expected in the natural scrolling convention
11. Disable Natural Scrolling again
12. Now Skim works as expected
13. Reboot Skim, it still works as expected! This is the workaround I mentioned.

Configuration
Skim Version 1.7.12 (164) (but I first noticed the problem in late December 2025 I think)
Mac OS Tahoe 26.2 / Macbook Pro M3 and also Mac OS 25.something / Macbook Pro M1

Sorry for submitting a non-reproducible bug. Please let me know if I can provide any other information since presumably my other system still exhibits this behavior.

[Christiaan Hofman]

This is a duplicate of other scrolling bugs. Note that this is due to a system bug. We can not fix this, unfortunately.

[Christiaan Hofman]

BTW this happens when you have Always Show Scroll Bars (or perhaps also based on your mouse), and the view is less wide than the content (so the horizontal scrolling is shown.) Somehow the system code does not take into account of the width of the horizontal scroll bar, jumping up by its width in several event handling code. You can work around this by changing the scroll bar showing setting (e.g. set it to WhenScrolling), which can also be done just for Skim using a hidden preference.

[Christopher Rooney]

Thanks! I didn't realize this was the same bug as the "document scrolls when selecting text" and others since I hadn't noticed those behaviors.

I found the hidden preferences page, but WhenScrolling is not listed as an allowed option for the AppleShowScrollBars. Nevertheless, using defaults write -app Skim AppleShowScrollBars -string WhenScrolling did the trick!

https://sourceforge.net/p/skim-app/wiki/Hidden_Preferences/#h-system-overrides

[Christiaan Hofman]

Sorry, the description of the values was swapped with another option. That is now fixed.