>>> My Info: Win7_64, Nightly 49, 32bit, ID 20160526082509 STR_1: 0.(unnecessary) Set "general.smoothScroll.mouseWheel.durationMaxMS" to 4000 to notice the bug better 1. Open http://www.kp.ru/daily/26592/3608653/ 2. Scroll the left sidebar on the page to the bottom, so that only ~half of scrollbar thumb is visible 3. Hover mouse over the empty area in scrollbar (i.e. not scrollbuttons and not scrollbar thumb) 4. Rotate mouse wheel up once, wait until scrolling is finished 5. Repeat Step 4 several times AR: Scrollbar thumb redraws with delays (Scrollbar smoothly moves to the top, and sometimes it appears clipped for ~0.5s. Then it redraws, and the whole visible area of scrollbar thumb is rendered fine. It is similar to page checkerboarding ER: No glitches in scrollbar thumb
I can repro minor checkerboarding and artifacts on the scrollbar. It looks like it is getting drawn as it scrolls into view, and there are lines on it dividing the old segment from the newly-drawn segment. The lines go away if I do something that causes the entire thumb to get redrawn. No idea what's causing this. I'm not even sure if the scrollbar is a OS scrollbar or created by the page. I suspect the latter because of the auto-hiding behaviour it has but I can't find the element in the inspector.
Created attachment 8824754 [details] testcase 1 - bug 1327932.zip Attached testcase works better as a testcase, because it doesn't change length of the sidebar.