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Even if nsMouseScroll event has larger delta than the target frame size, Gecko scrolls it just one page because if some lines are skipped, the user may not find the lines. This is especially serious problem on listbox.
Stating with Mozilla 19 (Firefox 19) or later, this limitation is ignored when the delta_multiplier_* pref for the event value equals or is larger than 100000 (x1000). This allows users to scroll to start or end of the page with only one manipulation.
= Minimum scroll amount at scrolling by line =
When a wheel event whose deltaMode indicates line scroll scrolls a content, nsEventStateManager refers the line height of the content. However, if the line height is zero, e.g., font-size: 0; is specified, scroll is never performed.
Against this issue, Mozilla 17 (Firefox 17) or later supports '''mousewheel.min_line_scroll_amount''' pref. If line height of the scroll target is smaller than this value in pixels, a wheel event which represents one line scroll scrolls this amount.
However, this pref doesn't affect to the DOM event's delta values. This is used only at handling the default action.
= Preferences for customizing delta values and default action =
