For example;

<div style="height: 300px; overflow: scroll;">
<div style="height: 5000px;"></div>
<iframe fission="true" ...></iframe>
</div>

above case we don't generate the display item for the iframe, so we don't call UpdateEffects() at all.

Attaching file is a mochitest to see the issue easily.

:mattwoodrow, where is the right place to call UpdateEffects with FullyHidden EffectsInfo?

Beware of me trying to change the default visibility of docshells in bug 1572798, though there's some orange I haven't had the chance to go back to.

But It may be a better way to handle this...

Oh nice. That's good to know. Setting inactive in the first place seems to work at least for out-of-process shells.

After some more thought, the example in comment 0 is not appropriate. Even if the iframe is far from the display port boundaries, the iframe's doc shell should be active. But if the iframe is under a deck pack which is not selected in the first place, the iframe's doc shell should be inactive, I think. It's the case in bug 1541256.

Maybe I am wrong. It's not yet clear to me what the active means, if it means whether the doc shell is visible or invisible, the example in comment 0 is still applicable.

Hiro, is this bug still a problem?

If this bug as been fixed since you reported, should we land the new mochitest you posted in comment 0?

Looks like now the test works fine. Yeah, it's time to land it.

In that case, we'll track this bug for Fission Nightly (M6).

https://hg.mozilla.org/mozilla-central/rev/f588a463cc5a