dom: make SafeTriangle smaller, add tests (#224401)

* debug: improve behavior for slow-running prelaunch tasks

Shows this notification if a prelaunch task has been running for more
than 10 seconds:

![](https://memes.peet.io/img/24-07-be15cd9d-c9cd-40dc-8937-9d62466e5c96.png)

Also does a little cleanup of some probably-leaking disposables in the task runner.

Fixes #218267

* hygenie

* dom: make SafeTriangle smaller, add tests