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/#review193228

Still r-, because I was surprised to see
"expected: kScrollDown | kScrollLeft" in case there is both vertical and horizontal scrolling with modifier.
Please explain or fix.

::: dom/events/test/window_wheel_default_action.html:1830
(Diff revision 2)
> +
> +  const kTests = [
> +    // Without modifier key, the default's action should be used.
> +    // I.e., vertical scroll is expected normally.
> +    { defaultActions: {
> +        default: 1, with_shift: 2, with_control: 3

What these numbers are? 1, 2, 3? Please add some comment

::: dom/events/test/window_wheel_default_action.html:1853
(Diff revision 2)
> +               deltaX: 0, deltaY: -1.0,
> +               lineOrPageDeltaX: 0, lineOrPageDeltaY: -1,
> +               shiftKey: false, ctrlKey: false, altKey: false },
> +      expected: kScrollUp
> +    },
> +    // If only the modifier to treat vertical wheel as horizontal scroll,

I don't understand this sentence.

::: dom/events/test/window_wheel_default_action.html:1914
(Diff revision 2)
> +      deltaMultiplierX: 1.0,
> +      deltaMultiplierY: 1.0,
> +      event: { deltaMode: WheelEvent.DOM_DELTA_LINE,
> +               deltaX: 0, deltaY: 1.0,
> +               lineOrPageDeltaX: 0, lineOrPageDeltaY: 1,
> +               shiftKey: true, ctrlKey: true, altKey: false },

Oh, modifier in the JS object isn't about the event but about the pref. Could you document that somewhere

::: dom/events/test/window_wheel_default_action.html:1981
(Diff revision 2)
> +               deltaX: 0, deltaY: -1.0,
> +               lineOrPageDeltaX: 0, lineOrPageDeltaY: -1,
> +               shiftKey: true, ctrlKey: true, altKey: false },
> +      expected: kScrollUp
> +    },
> +    // If the wheel is operated diagonally, shouldn't treat the deltaY as

This sounds wrong. If the pref is set, and modifier is pressed, why should viewport scroll ever horizontally? I don't see such behavior in Chrome on Windows

::: dom/events/test/window_wheel_default_action.html:2184
(Diff revision 2)
>      ["mousewheel.default.action.override_x", -1],
>      ["mousewheel.with_shift.action", 2],   // history
>      ["mousewheel.with_shift.action.override_x", -1],
>      ["mousewheel.with_control.action", 3], // zoom
> -    ["mousewheel.with_control.action.override_x", -1]]},
> +    ["mousewheel.with_control.action.override_x", -1],
> +    ["mousewheel.modifier_to_treat_vertical_wheel_as_horizontal_scroll", 0]]},

I don't understand this. Why 0? Doesn't that disable the feature.

::: modules/libpref/init/all.js:2759
(Diff revision 2)
> +// to one of 16 (Shift), 17 (Ctrl), 18 (Alt/Option) and 224 (Command), the
> +// corresponding modifier key with vertical scroll is treated as horizontal
> +// scroll.  Note that this setting may override the action set by
> +// "mousewheel.with_*.action" and mousewheel.with_*.action.override_x.  E.g.,
> +// when this is set to 16 (Shift) and "mousewheel.with_shift.action" is set to
> +// 2 (History Navigation), try to scrolling vertically with Shift key causes

s/try to//

::: widget/MouseEvents.h:597
(Diff revision 2)
>               mLineOrPageDeltaX : mLineOrPageDeltaY;
>    }
>  
> +  void SwapDeltaXAndDeltaY()
> +  {
> +    auto tmpDelta = mDeltaX;

I guess you could use std::swap here