Backstory in bug 1328285 comment 13 onwards. It looks like with e10s disabled on Firefox (v51 or older, for example), a user on a Windows touch device could drag items on http://html5demos.com/drag simply by putting their finger down on an item and dragging. With e10s enabled that functionality stopped working. Note that this is independent of APZ/touch event support - that is, the regression is caused by e10s and not by APZ or touch events, because the regression manifests even with layers.async-pan-zoom.enabled=false and dom.w3c_touch_events=0. I'm not really sure if this is something we can actually even fix now that we have touch event support, since I presume that the tag+drag gesture was generated by the OS and will not be generated now that we handling touch events ourselves. If we really need to fix it we might be able to by reimplementing the functionality in APZ but it would likely be expensive (both in terms of coding time and perf) because of the tricky interaction with other gestures.
In a sense this is similar to bug 1187439 in that it was a touch thing regressed by e10s. Unfortunately APZ doesn't fix this one out of the box.
See Also: → bug 1187439
Also for the record both Petruta and I are running the Windows Insider Preview, and I don't recall seeing tap/drag working on the current release version of windows 10. I'm 95% sure this is a new Windows feature as I feel I would have noticed it before if it had been in the non-preview build. So technically this regression may never be observed by users, unless they are using a non-e10s build through the next windows update.
You need to log in before you can comment on or make changes to this bug.