[Bug 1228250]

[R-bugs-h]

Comment on attachment 8915954
Bug 143038 Make users can scroll contents horizontally with vertical wheel operation with a modifier

https://reviewboard.mozilla.org/r/186794/#review195394

::: browser/app/profile/firefox.js:639
(Diff revision 4)
> -pref("mousewheel.with_shift.action", 2);
> +// only vertical wheel but want to scroll horizontally.  For such users, we
> +// should provide horizontal scroll with shift+wheel (same as Chrome).
> +// However, shift+wheel was used for navigating history.  For users who want
> +// to keep using this feature, let's enable it with alt+wheel.  This is better
> +// for consistency with macOS users.
> +pref("mousewheel.with_shift.action", 4);

ok, this is of course a change to the default behavior, but we do have
some time to get feedback

::: dom/events/EventStateManager.cpp:3298
(Diff revision 4)
> -      // When APZ is enabled, the actual scroll animation might be handled by
> -      // the compositor.
> -      WheelPrefs::Action action;
>        if (pluginFrame) {
>          MOZ_ASSERT(pluginFrame->WantsToHandleWheelEventAsDefaultAction());
>          action = WheelPrefs::ACTION_SEND_TO_PLUGIN;

You change the ordering of whether action is first checked for plugin or
apz, and delta is adjusted before the plugin chcek. But I guess that
makes sense. But please test (manually) some Flash doing scrolling.

::: dom/events/EventStateManager.cpp:5915
(Diff revision 4)
>  
> -  *aOutMultiplierX = mMultiplierX[index];
> -  *aOutMultiplierY = mMultiplierY[index];
> +  // If the event should be treated as horizontal wheel operation, deltaY
> +  // should be multiplied by mMultiplierY, however, it might be moved to
> +  // deltaX for handling default action.  In such case, we need to treat
> +  // mMultiplierX and mMultiplierY as swapped.
> +  double multiplierForDeltaX = mMultiplierX[index];

We do have this similar code in several places.
Can you think of anyway to have a helper method to do this all? If not, fine.

::: dom/events/WheelHandlingHelper.h:233
(Diff revision 4)
> +  WidgetWheelEvent& mWheelEvent;
> +  double mOldDeltaX;
> +  double mOldDeltaZ;
> +  double mOldOverflowDeltaX;
> +  int32_t mOldLineOrPageDeltaX;
> +  bool mTreatedVertualWheelAsHorizontalScroll;

Vertually? Do you mean Virtually

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https://bugs.launchpad.net/bugs/1228250

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

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