Before that we were not notifying the image frame in any way if we ended up not
doing a load, and we were instead relying on the reflow the viewport resize
caused to get the new density in ComputeSize from the content node (but nowhere
else, since that's the bug part 1 fixes).
This was generally unsound, since you can stash random media in a sizes=
attribute, which don't necessarily needs to cause a reflow.
Now we need to notify necessarily because nsImageFrame stores the adjusted
intrinsic size.
mCurrentDensity could also get out of sync as well, when the selected image
density changed, but we ended up returning early because our source hadn't
change in the first early exit.
This patch moves us to a model where we don't re-trigger loads for density
changes if the source doesn't change (unless we pass aAlwaysLoad when we need
to, per spec).
This matches our previous behavior (without the bugginess of not updating the
intrinsic size), and also Chromium, at least.
This changes behavior in one case, which is when we don't load the same source
node, but we have the same source URL, and the density does change. This could
happen with <picture> and two <source>s with same source and different media and
sizes. This makes our behavior consistent with the behavior we have when both
the source and the density doesn't change.
Blink and WebKit do trigger a second image load both when the source changes
without changing density and when density changes. I'll file a spec issue, since
per:
https://html.spec.whatwg.org/#reacting-to-environment-changes
We should be triggering the load when the density changes but the source
doesn't as well, but no UA does that.
I filed https://github.com/whatwg/html/issues/3709 with a little summary of the
situation and what I think the behavior should be (which is what this patch
implements). That being said, I'll update the impl if the spec people think
otherwise :).
MozReview-Commit-ID: Eqy16ygHRLo
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