Is there a reason you're still running Mavericks (OS X 10.9)? As far as I know, any machine that can run Mavericks should be able to run El Capitan, which is a free upgrade. No idea if that will fix this, but that'd be the first thing I'd try. I can't recall anyone else reporting this.
If for some reason you cannot run El Capitan and your scroll bar is flickering irregularly you can fix this by turning off transparency, restarting and (if you want) turning transparency on again.
I don't have the ability to read your problem report so I cannot see what Zotero version you are using. I do remember that there was a FireFox plug-in (or maybe it wasn't FireFox but a Mac utility) that used an on/off blink of the scroll bars as an alert. I quickly removed it because I found it annoying.
Upgrade Storage
Or, is this what you see:
https://github.com/codemirror/CodeMirror/issues/2897
I don't have the ability to read your problem report so I cannot see what Zotero version you are using. I do remember that there was a FireFox plug-in (or maybe it wasn't FireFox but a Mac utility) that used an on/off blink of the scroll bars as an alert. I quickly removed it because I found it annoying.