HAve you somehow disabled the overlay scrollbars? When you hover the mouse cursor near the scrollbar area you should see a larger scrollbar handle (with up & down buttons) appear next to it. The tiny scollbar visible in your video is just a visual indicator, not the bit you are expected to use to scroll with.
Try running this in a terminal:
Code:
gsettings reset com.canonical.desktop.interface scrollbar-mode
If that doesn't do it, make sure the overlay-scrollbar package is installed, and try specifically setting the scrollbar mode:
Code:
gsettings set com.canonical.desktop.interface scrollbar-mode overlay-auto
(or to force normal scrollbars instead
Code:
gsettings set com.canonical.desktop.interface scrollbar-mode normal
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