This patch declares a new default action, "horizontal scroll", this scrolls
content horizontally with deltaY of wheel events and ignores deltaX and deltaZ.
This is used for default action with Shift key in default setting except on
macOS. On macOS, legacy mouse's vertical wheel operation with Shift key causes
native horizontal wheel event. Therefore, we don't need to use this new
default action on macOS. Additionally, old default action with Shift key,
navigating history, is moved to with Alt key. This makes same settings between
macOS and the others. So, this is better for users who use macOS and another
OS and web app developers who check wheel events only on macOS or other
platform(s).
For simpler implementation, default action handlers moves deltaY values to
deltaX values temporarily *only* while they handle wheel events. This is
performed by AutoWheelDeltaAdjuster and restored after handling it
automatically.
So, in other words, even if default action is "horizontal scroll", web apps
receives wheel events whose deltaY is not zero but its content will be
scrolled horizontally. This is same as Chromium, so, this behavior shouldn't
cause any incompatible behavior with it.
MozReview-Commit-ID: E4X3yZzLEAl
--HG--
extra : rebase_source : e20d854c6b0a181ad4c9e7304bd9ad14256481ff
