I'm experiencing the same issue on Ubuntu 18.04. Though the appearance is different. The scroll marker (in my vanilla Ubuntu install it's orange colored) is not properly cleared on movement and eventually the whole vertical scroll bar turns orange.
I'm not uploading a screenshot, but here's how I can repro this.
- open a file (I opened a C file) - then open a Search > Find (Ctrl+F) dialog - then find something (I typed 'gchar' as I've quite a few of those) - once it matched one, with Find dialog still on-screen, try to manually drag the scroll marker in Editor window. - the scroll marker gets "smeared" all over the scroll bar area.
I have no plugins enabled.