Fix initially suspended component under sCU

[sventschui]

So the parking approach to suspense has an issue when the suspending component suspends on the first render (as lazy() does) and is under a component with an sCU that returns false. This is represented by the newly added should work throughout sCU when initially being suspended test.

My first approach was to patch the _parentDom attribute and then call forceUpdate() on the component that suspended. The problem with this is, that when this component is directly under Suspense that it will show up (modify the DOM) too soon.

The second approach (this PR) is to not patch the _parentDom but call forceUpdate() on the _ancestorComponent of the suspending component. This seems to work but feels wrong. It also breaks other tests since it will result in some render methods being called too often.

Other thoughts I will explore:

• Patch _parentDom and call forceUpdate on all suspending components once all of them finished. This seems wrong as a suspending component might suspend again.

[ForsakenHarmony]

guess it's similar to what we do for context

https://github.com/preactjs/preact/blob/master/src/create-context.js#L25

[sventschui]

Yes it's similar but with the issue that we want to "postpone" the update of the dom but not the re-render because:

• the update of the dom will result in the fallback being overriden if there is no DOM between <Suspense> and the suspending component
• calling render as soon as possible is crucial as it might suspend multiple times