#1599 Scrolling direction inconsistent depending on zoom level

[dominik]

When viewing a PDF at certain zoom levels, scrolling behavior intermittently responds in the wrong direction. The issue affects both trackpad/mouse scrolling and keyboard arrow navigation. The behavior feels inconsistent and disorienting.

Steps to Reproduce

1. Open any PDF file in Skim.
2. Zoom in to a higher magnification level (e.g., significantly above 100%).
3. Scroll using the trackpad or mouse wheel.
4. Alternatively, use the Up and Down arrow keys to navigate.

Actual Results

• Scrolling sometimes moves partially or momentarily in the opposite direction of the input.
• After zooming in, pressing the Up arrow scrolls up as expected, but pressing the Down arrow can also produce movement that feels partially upward or inconsistent.
• The scrolling behavior feels unstable or reversed depending on the zoom level.

Expected Results
Scrolling direction should consistently match user input regardless of zoom level.

• Up input should always scroll up.
• Down input should always scroll down.
  Zoom level should not affect scroll direction behavior.

Configuration

• Skim version: Version 1.7.12 (164)
• macOS version: 26.3 (25D125)
• Mac model and architecture: M3 Max

Additional Information
The issue appears to be triggered specifically after zooming in. It does not consistently occur at default zoom levels. The problem affects usability due to the unpredictable scrolling direction.

If you cannot reproduce the behavior, I can provide a screencast demonstrating the issue. I believe the problem first appeared after the most recent update.

[dominik]

Addtional Information: It does also happen if the app is in fullscreen mode.

[Christiaan Hofman]

This is a duplicate of bug # 1591. When you always show the scrollbars (in your system preferences) and the horizontal scroll bar is visible (which will happen when you zoom in), the pdf content is scrolled upwards by the width of that scrollbar on various events (such as scrolling actions, but also mouse actions). Unfortunately as the bug must be in the system frameworks (as our code is not involved at this point), there is nothing we can do about this.

You can work around this bug by turning off always showing the scrollbars. This can be done by a hidden preference for Skim alone. See the bug 1591 comments or the Wiki for details on this hidden preference.

[dominik]

Thank you for the fast response. It’s a shame that Apple cannot fix their framework. But you fix helps! :)