The "Common smart indent macros" window appears to be redrawing improperly by repainting the area where the text input area is located with the text area background color inadvertently wiping the textarea out, then drawing the entire textbox on top of this repainted area, causing a flickering effect. This redraw of the entire textbox area is performed for each and every line scrolled.
Open the "Program smart indent macros" window by going to Preferences -> Default Settings -> Auto Indent -> Program Smart Indent...
dropdown the Language Mode box and, unless you have another set of indent macros installed, I recommend you select C++, as its initialization code is fairly substantial.
Click the "Common / Shared Initialization..." button.
Scroll the textarea in the window that has just opened up and down relatively quickly, and observe the behavior I have described above.