[webkit-dev] Proposal: Add support for focus rings in Canvas 2d

Ryosuke Niwa rniwa at webkit.org
Mon Oct 14 14:35:18 PDT 2013

On Mon, Oct 14, 2013 at 2:28 PM, Rik Cabanier <cabanier at gmail.com> wrote:

> On Mon, Oct 14, 2013 at 1:31 PM, Timothy Hatcher <timothy at apple.com>wrote:
>> On Oct 14, 2013, at 12:43 PM, Rik Cabanier <cabanier at gmail.com> wrote:
>> Also, how would your suggestion tell the UA about what areas are
>> associated with the elements? What happens if an element is no longer
>> focused? The ring is drawn into the canvas bitmap so those pixels have to
>> be regenerated.
>> Focus rings are usually larger than the control they surround. How is the
>> developer suppose to know the pixel padding needed for each platform's
>> focus ring? Guess and hope for the best?
> Why would he need to know this? Is it for the path that describes the ring?

Isn't focus ring drawn on the canvas?  If so, it's important the focus ring
fits within the canvas. e.g. consider focusing an element of 100px by 100px
inside a canvas of the same size.  If the focus ring were to be drawn
around the element that currently has focus, the entire focus ring would be
drawn inside of the visible region.

Would drawing the system focus ring taint the canvas pixels? (Drawing form
>> controls into canvas via SVG images and <foreignObject> has been considered
>> taint worthy because it could leak the user's UI theme.)
> I'm unsure if it should taint the canvas. How much information would be
> leaked that isn't already available through other means?

It may, for example, leak information as to whether user's machine in high
contrast mode, which is another dimension for finger-printing user.

- R. Niwa
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