Bug 441088

SUMMARY
Clicking on scrollbar tracks (i.e. empty space above or below the scrollbar) with the primary button in plasmoids (widgets) and panels causes the scrollbar to jump to the clicked location irrespective of the "Clicking the scrollbar track" setting in System Settings.

STEPS TO REPRODUCE
1. Open System Settings
2. Go to Workspace Behavior > General Behavior (should be automatically opened if using sidebar view)
3. Select "Clicking the scrollbar track: Scrolls one page up or down" and apply changes
4. Click on the scrollbar track (blank space) in any plasmoid or the Add Widgets panel. I recommend to try this on the Clipboard plasmoid, since it's very easy to populate it with items.

OBSERVED RESULT
The scrollbar jumps to the clicked location, and contents would scroll to that spot accordingly. Scrolling contents with a click of the middle mouse button also fails.

EXPECTED RESULT
When the above setting is applied, the scrollbar and contents would scroll one page up or down, and the middle mouse button moves the scrollbar to the clicked locations.

SOFTWARE/OS VERSIONS
Operating System: Manjaro Linux
KDE Plasma Version: 5.22.4
KDE Frameworks Version: 5.85.0
Qt Version: 5.15.2
Kernel Version: 5.13.11-1-MANJARO (64-bit)
Graphics Platform: X11
Processors: 12 × AMD Ryzen 5 1600 Six-Core Processor
Memory: 15.6 GiB of RAM
Graphics Processor: Radeon RX 570 Series

ADDITIONAL INFORMATION
For most users, the primary mouse button is configured to the left mouse button. In addition, the "scroll one page up or down" setting is the default; therefore, the described result above defeats that purpose.

This behavior persists across Plasma sessions and reboots. The said behavior is not new to Plasma 5.22, as I've seen this exact behavior in older versions of Plasma.