This issue has evolved into a question of the best way to spec external texture expiry. See discussion below.

Original issue summary

This is probably a fairly minor issue, but deserves to be raised in the group because I misrepresented it in meetings due to my own misunderstanding of how the HTML event loop runs and how it relates to the production of frames. @toji and I found this while working through another spec issue.

In very, very broad terms, the event loop is:

• I thought: loop { run a bunch of tasks, update rendering of visible docs }
• Actually: loop { run one task, update rendering of docs which are due for an update }

The result of this is:

• I thought: stale external textures expire once per frame, just before any document updates if any.
• Actually: stale external texture expire after every single task (but only for top-level tasks, not things like rAF callbacks).

I think that what we ended up speccing actually makes good sense, so I propose we should keep it approximately as is (with possible smaller changes that tie into another issue, #3295).