SUMMARY Attempting to use the built-in Windows horizontal scroll function (holding shift then scrolling with a mouse wheel) results in vertical scrolling. STEPS TO REPRODUCE 1. Open a PDF Document 2. Zoom until the document is able to be scrolled horizontally 3. Hold "Shift" Key and use mouse scroll wheel OBSERVED RESULT Document scrolls vertically. EXPECTED RESULT SOFTWARE/OS VERSIONS Windows: Windows 10 (available in About System) KDE Frameworks Version: 5.63.0 Qt Version: 5.13.1 ADDITIONAL INFORMATION Horizontal Scrolling in Windows is a known pain to deal with, so it is likely something in the windows-specific connections in the port for Okular that the issue is being introduced. I have not yet verified if touchpad 2-finger scrolling allows for horizontal scrolling (which is the case in some programs with this issue, ie Excel). In fact, it is possible that the dll patching methods that users have put together for Excel (see the OfficeScroll project here: https://github.com/T800G/OfficeScroll) could give an indicator of how to easily allow for this to be fixed.
In Qt apps (all of them, not just KDE apps), it's Alt+scroll, not Shift+scroll. Yes, this is different from how it is in native Windows and macOS apps, both of which use Shift+scroll. Yes, this is annoying. It annoys me too. However if it's to be changed, it would need to be done in Qt itself, not in Okular. If you'd like to pursue the matter further, please file a bug for the Qt developers at https://bugreports.qt.io/ and add it to the "URL" field of this bug report. Thanks!
Apologies for the *very* late response, but just wanted to say thank you for pointing out this functionality to me. I had not been able to surface this in my searching, so while I will attempt to follow up with the QT developers, in the meantime this will be a great workaround for me.