Define timing of script execution in relation to DOM mutations better

[annevk]

See http://software.hixie.ch/utilities/js/live-dom-viewer/?saved=4098.

I think this is most clearly demonstrated when inserting a DocumentFragment node, but maybe there are similar scenarios I don't know about. Basically, the elements are inserted first, one by one, and then executed, one by one. This requires looping over the inserted elements twice. Probably once with a "disable script flag" and once without.

@smaug---- might have some ideas around designing this in a different way.

This came out of review in #1100 which indicated that currently this is not really defined properly at all.

[annevk]

This seems hard to fix. We cannot just run "insertion steps" twice since there'd be too many side effects for other elements. The simplest thing I can think of is keeping track of inserted HTML/SVG script elements and then invoking their insertion algorithm at the end of https://dom.spec.whatwg.org/#concept-node-insert.

[domenic]

Might it be worth finding out how implementations work here? @nyaxt or @hiroshige-g could probably help from the Chrome side.

[annevk]

I looked at Firefox's code in whatwg/dom#575 and it seems any notifications happen after all nodes are inserted, which seems reasonable. I haven't quite investigated if any scripts executing at that point can mess things up though.

(I also found that Safari is alone in reporting different numbers for the test, but hopefully they're willing to align.)

[rniwa]

In WebKit, we create a queue of "tasks" (e.g. firing events, adding a new stylesheet) while inserting nodes, and then execute those items in the queued order. There is an interesting race condition here in the case where those inserted nodes are subsequently removed in one of the "tasks". I think we'd just go ahead and execute all the queued "tasks". I wonder what Gecko & Edge do in those cases.

[smaug----]

In WebKit, we create a queue of "tasks" (e.g. firing events, adding a new stylesheet) while inserting nodes

FWIW, that sounds very similar to Gecko. We call these tasks ScriptRunners.