Why does <script> tag contained in <template> force MutationObserver to return early?

[miragecraft]

I'm investigating the behaviour outlined in #35393 and I don't understand the "correct" behaviour.

Sure, if a <script> runs it should trigger microtask cleanup, but a <script> tag contained within <template> does NOT run.

It makes no sense in this context.

I want to make changes to <template> tags synchronous via MutationObserver and this weird behaviour kneecaps that effort because I'm not guaranteed to have all template content available if it contains any <script> tag.

The fact that <script> tag within <template> triggers microtask cleanup should be considered a bug, not a feature.

[JRJurman]

I discovered this behavior recently, and was able work around it by checking if there were TEXT nodes at the end - details here: https://stackoverflow.com/a/77115327/864715