STR: (Win7_64, Nightly 43, 32bit, ID 20150907030206, new profile, safe mode) 1. Open URL 2. Place mouse in the center of content area 3. Click middle mouse button 4. Move mouse 1-5px below the autoscroll circle Result: Page is being scrolled, but elements repaint only when I move mouse over them Expectations: Page should repaint normally Workaround: 1) html5.offmainthread -> false 2) layers.offmainthreadcomposition.async-animations -> false 3) layers.offmainthreadcomposition.enabled -> false 4) Restart browser

Does turning off e10s or APZ (layers.async-pan-zoom.enabled) also stop this from happening?

As you can see on the video, e10s is not enabled. APZ doesn't stop this. // You asked _me_ because you failed to reproduce this yourself, right?

Correct.

(In reply to Markus Stange [:mstange] from comment #3) I saw you're using Mac OS. That's important information and should've been mentioned.

I figured out that "layers.offmainthreadcomposition.enabled"->false is enough to cure it. Markus, we have a theory that VM blocks/doesn't allow OMTC. (See bug 1189438 comment 13)

I created user.js in my profile where I enabled OMTC (just in case) and bisected using that profile. This was caused by bug 1165185. Regression range: > https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=cd741d3ae78a7ef127b6baf7c4efc0fff0472267&tochange=f7fca0232ffc4ec5855557cfb7a6d039b28488e4

Any success reproducing this bug on Windows?

This is the same underlying bug as bug 1209100 - we're failing to recomposite at the bottom end of very large pages.