[Bug 1228250]

[Masayuki]

Comment on attachment 8369358
Horizontal Scrolling with Mouse wheel+ modifier key

I believe that you want to swap the scroll direction completely.
However, your patch just swap the scroll direction of default action of
wheel event. E.g., scrollable element implemented with JS is scroll
normally with this patch. You need to swap the delta values before
handling all of them.

I.e., You need to swap them in PreHandleEvent() (after applying
delta_multiplier prefs?). And also you need to swap lineOrPageDeltaX and
lineOrPageDeltay at same time. Finally, you might need to swap
overflowDeltaX and overflowDeltay at the end of PostHandleEvent().

Additionally, you have to test new behavior with automated tests:

* Add new function to test if deltaX/deltaY are swapped in DOM event level.
http://mxr.mozilla.org/mozilla-central/source/dom/events/test/test_dom_wheel_event.html?force=1

* doTestScroll() should be run again with swapped pref. And also you need to test overflowX/Y are swapped correctly.
http://mxr.mozilla.org/mozilla-central/source/dom/events/test/window_wheel_default_action.html?force=1

* Please test it with native scroll event (this is now only available on Windows)
http://mxr.mozilla.org/mozilla-central/source/widget/tests/window_mouse_scroll_win.html?force=1

However, I don't think that this is good patch because this may make the
code difficult to maintain. Therefore, basically, I don't agree with
adding this feature suggested by this bug. How many people still want
this feature? In these days, most pointing devices support horizontal
scroll with tilt wheel or swipe gesture. Why this isn't enabled in
default settings? These fact means that this makes the code complicated
only for a few users. I'm not sure if this is worthwhile to fix this
bug. I want to suggest this bug marked as WONTFIX. Could you explain why
do we need to fix this bug even with such cost and appending risk?

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Title:
  [Shift + Mouse-Scroll-Wheel] Does NOT Scroll Horizontally

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