Created attachment 8661345 [details] video - Everything twitches if I autoscroll web page on zoom level is less than 100percent.webm STR: (Win7_64, Nightly 43, 32bit, ID 20150913030204, new profile, safe mode) 0. Set pref layout.css.devPixelsPerPx -> 1.0 (because on HiDPI it reproducible for another number than 100% - less than 100%) 1. Open https://en.wikipedia.org/wiki/Shadow_Raiders (in fact any page exposes this behavior) 2. Reduce zoom level so that it was < 100% (e.g. 70%) 3. Click middle mouse on free space on page to start autoscrolling 4. Move mouse 1-5px below the autoscroll circle Result: Everything is twitching. This is safe mode. Expectations: Page elements shouldn't move relative to each other.
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Firefox/45.0 Managed to reproduce this in normal conditions (safe mode is not mandatory) on latest Nightly (45.0a1) with the above mentioned steps. Updating the issue component.
Component: Untriaged → Panning and Zooming
I think this is just rounding changes in the positioning of elements. 70% is going to result in fractional numbers when positioning. Moving to layout.
Component: Panning and Zooming → Layout
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