Can confirm a very randomly appeared flickering. Non other open program with scrollbars does it only Geany. My setup - Debian Testing, XFCE, breeze-gtk-theme.
Scrollbar appears to be flickering from top to bottom, it may start at middle, but it will go down. Blinking about 1-5 centimeters at a time, second blink appears a little lower, not necessarily exactly below the first, but it may be that way also. Currently it blinks in 2 of 3 tabs open. All files are saved at my home directory.
Tried to record a gif - flickering is not visible there, don't have any amateur cameras at hand, and not sure if this would be appropriate to attach.