In almost all applications that I use regularly I can scroll sideways by holding Shift while scrolling, Dolphin is the only exception. In all KDE applications, holding Alt seems to be the standard. And I couldn't find any way to change that. I think a global option in the KDE System Settings would be the best idea.
Alt instead of Shift is Qt-ism, so we've inherited it by using Qt as our GUI toolkit. I 100% agree that this is odd because every other app toolkit I'm familiar with uses shift instead. However this isn't something we can easily override in KDE land. Could you report it to the Qt folks? Thanks! https://bugreports.qt.io/
I think someone else will have to make the feature request on the Qt bug reports page, I wouldn't know what to ask for. But I just found out that it must be possible to do horizontal scrolling in Qt applications with the Shift key because in the Falkon browser both Alt and Shift key work for horizontal scrolling by default.
Please don't reset the status from UPSTREAM to FIXED. UPSTREAM is the correct status for a bug that needs to be fixed in a library used by KDE software. See https://community.kde.org/Get_Involved/Bug_Reporting#Understand_what_the_resolution_statuses_mean Falkon probably decided to override this in the interests of usability. That's fine for Falkon, but the best solution is to do it for everyone because it's not really feasible to ask every KDE app (there are hundreds) to make the same workaround. I've filed a Qt bug: https://bugreports.qt.io/browse/QTBUG-75949
Thank you.