+++ This bug was initially created as a clone of Bug #574663 +++ This is just like bug 574663, but first you need to flip "mousewheel.enable_pixel_scrolling" to false in about:config. It looks like the patches in bug 574663 only modified the pixel-scrolling codepaths.... I finally figured out today that this is what causes sites to randomly zoom for me.
Created attachment 549815 [details] [diff] [review] Don't do zoom or history actions for momentum scrolls, no matter what's going on with pixel scrolling.
You could extend content/events/test/test_bug574663.html to test this, for example by adding another flag to sendTouchpadScrollMotion that makes it send one line scroll with hasPixels = false instead of what it currently does.
Hmm. Let me give that a shot!
Created attachment 549820 [details] [diff] [review] With the test change Verified that the changed test fails without the code change and passes with it.
Comment on attachment 549820 [details] [diff] [review] With the test change r=me. # Inside the if's parentheses are very messy, though... I'll sort out them ASAP.