When the document itself changes a new docState will be created, but if the page is using the history API then the documentURI can change without the need to create a new docState.

If the matcher has a path then this could mean the position is stored for all subsequent videos on that domain.

Example:

1. Have this matcher: "https://*.twitch.tv/mikeconley_dot_ca/*": TOGGLE_POLICIES.TOP,
2. Go directly to a video that matches that (open a new tab and put the URL in).
3. Go to some other video on twitch.

Expected Results:
The PiP button is in the default position.

Actual Results (or, expected actual results?):
The PiP button's position is still cached since it's only set once per document.

Yeah, this is a problem. Good catch. We should probably get this patched and maybe uplifted before we try putting any policies into the webcompat add-on.

Does this issue reproduce now that we have overrides? I haven't seen it myself.

I actually think this is the root cause behind the Netflix toggle bug. I believe if users land on the Netflix browse page, the "hidden" policy is applied to all videos (per policy here: https://searchfox.org/mozilla-central/rev/26b13464c2beb26e0d864d561c30e817a85c348a/browser/extensions/webcompat/data/picture_in_picture_overrides.js#32), but then upon choosing a video, the history API is used instead of a full page navigation. This causes the policy to "transfer" even though the URI has changed.

https://hg.mozilla.org/mozilla-central/rev/7da47d42204e