Touchpad gestures can generate much smoother scrolling events than the
discrete increments of mouse wheels. GDK3 supports this by means of a
new kind of scroll event, with direction GDK_SCROLL_SMOOTH and a
floating-point delta value. Added support for this; so in GTK3 mode,
you can now touchpad-scroll at a granularity of one line rather than
five, but in mouse tracking mode, scroll events are still grouped into
5-line chunks for purposes of turning them into escape sequences to
send to the server.
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