I would expect that your problem is caused by a Theme or a Style. Could even be an extension. It should generally be rather obvious by looking at each list.

Disable the Theme and return to the Default theme via Tools > Add-ons > Appearances.

Disable "Stylish" extension from it's own toolbar button using "Turn all styles off"/"Turn all styles on" -- it does not affect the styles that you previously individually disabled, so turning all styles back on returns you to normal. If the problem when away with that figure out which style was responsible it would show up as a theme.

Check your addons.

Terminology: vertical scrollbar (or right-hand scrollbar), and the slider may be also be called a "thumb".

No theme, no stylish extension, no addons.

Try a different Windows theme.

Control Panel -> Personalization -> Themes
Control Panel > Personalization > Window Color and Appearance > Advanced

Thanks cor-el! That helped.

You're welcome