Steps to reproduce: 1. Set layers.async-pan-zoom.enabled = true 2 Make sure mousewheel.default.delta_multiplier_y = 100 (default) 3. Scroll page with mousewheel and note smoothness. 4. Change mousewheel.default.delta_multiplier_y = 101 (non-default) 5. Again scroll page with mousewheel and note smoothness. Expected results: APZ scrolling smoothness should be unaffected by the delta multiplier preference. Actual results: A non-default delta multiplier has a dramatic negative impact on scrolling smoothness, as if APZ is disabled. According to Bug 1146727, several mousewheel preferences are not supported by APZ, but mousewheel.default.delta_multiplier_y is not one of them. Being able to modify the mousewheel vertical scroll delta is important on Linux where there is no configurable system scroll delta. A workaround to increase mousewheel delta without losing APZ smoothness is to increase mousewheel.min_line_scroll_amount.

Hm I tried doing this on my Linux desktop and I don't really see any difference but it could be hardware-specific (or my eyes aren't good enough). Are you able to grab a screen capture of the issue you're seeing? CC'ing mchang because it might be interesting to try and write a test for this similar to bug 1077651.

It's more noticeable when the page is loading and I can reproduce it on Windows 8.1. 1. Enable APZ, restart, set mousewheel.default.delta_multiplier_y = 101 (non-default). 2. Maximize the window. 3. Go to youtube.com 4. While the page is still loading, quickly scroll the mousewheel repeatedly. 5. Note the way scrolling stutters as the page loads. It behaves exactly the same as with APZ disabled.

Video of issue: https://drive.google.com/file/d/0B9lmHX09V69teU41eHh1cVFNaE0

This might be fixed now, as of bug 1214170. Can you please retest on the latest nightly?

Confirmed fixed, thanks for the update.