1558433 - Improvements to the visible content heuristic during page load.

Status: NEW

(Core :: DOM: Core & HTML, enhancement, P3)

Description

[Emilio Cobos Álvarez (:emilio)]

The commit message for bug 1555133 mentions a few potential improvements that could be made to the heuristic introduced there. Quoting:

• Maybe have some more heuristics for hidden elements, like presence of the
  hidden attribute?

• Maybe skip whitespace-only text? This does a pretty good job anyways because
  whitespace nodes are usually pretty small (like a couple newlines and
  spaces), so they don't add too much to the number. This could be done cheaply if
  looking at sSpaceSharedString / sTabSharedString.

• Add some weight to some elements? Maybe images should have a fixed weight,
  for example. Though you don't want 0x0 images and such to count... Maybe we
  should add to this heuristic out of band when processing image loads or some
  such.

• Handle shadow DOM and such better? Right now Shadow DOM and XBL are always
  assumed visible as long as they're connected. You can in theory do
  something like stash a <div> inside a <style> element, attach a
  ShadowRoot and such, and append a bunch of stuff inside. But I don't think
  it's something we should be particularly worried about.

• Probably add some check to CharacterData::AppendText as well? Otherwise this
  undercounts when loading big amount of text arrives via the network, for
  example, but also I'm not sure we're optimizing for log files and such so it
  might be ok.

We should measure and tune it if there are bits of the heuristic which don't work great.

I think the whitespace stuff and the AppendText stuff are likely to be worth doing.