[webkit-dev] Proposal: Rect based HitTest for a better touch experience

Antonio Gomes (:tonikitoo) tonikitoo at gmail.com
Wed Jun 2 12:46:12 PDT 2010


As most of you have experienced, the precision of a mouse click on
Desktop applications, including Web browsers, is much higher than the
precision of a touch tap on a mobile device. It is not a new problem,
and many solutions have been proposed to improve that situation. One
of the common solutions is to make a small target’s tap-sensitive area
larger, invisibly, than the visible target itself. Of course, it would
not work on a mobile Web browser, since it'd end up breaking the
desired layout of any non-simple Web site.

Mozilla has addressed this problem by implementing the SmartTap
feature [1]. The solution is simple: a "nodesFromRect" method is
available for applications {fennec, firefox, etc}. As the name
implies, given a rect (corresponding to tapped area), it returns a
list of candidate "mouse clickable" target nodes for the tap. This
list is sorted by z-order, larger intersection area, most visited (in
case of links), etc. Call sites can then pick the first element on the
list, or use any different heuristic to choose the target.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=489127

After some initial research, mostly understanding the HitTest system
of WebCore, I am sure we can provide something similar as a cross
platform solution for the problem.

For the moment, my plan is to use better hit testing to make it easier
for users to click & focus hyperlinks and input fields. For that, the
HitTest system would have to be modified to receive a Rect as input,
instead of a Point. It would also have to hold a list of possible
target candidate nodes, instead of only a single target node as it
does currently. In deep, the many ::nodeAtPoint implementations would
have to be adjusted to work with a Rect, not a Point, and probably
renamed to something similar to nodesAtRect. Much of the current
HitTest logic in RenderLayer would not need changes, though.

To keep the current mouse behavior (which is clearly point-based), we
could just pass rects with width and height equal to 1, so a Rect(x,
y, 1, 1) would behavior equally or very similarly to Point(x, y).

Before going this way, I would like to know of any known problem or
show stoppers.


--Antonio Gomes

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