virtual-scroll: Fixes scroll gap with autosize

[amcdnl]

This PR fixes the scroll gap that occurs at the end of a auto-sized scrolled. This resets the range and creates a new sample based on whats in view so we aren't 'predicting' the size and ending up with a gap.

Demo of issue: https://stackblitz.com/edit/angular-soc77e

[mmalerba]

Good to know that this fixes it, probably will want to find a way to do it that doesn't lose our size info though.

[amcdnl]

Ya, I suspected this was more of a starting point rather than the actual fix.

[nasreddineskandrani]

we need to see the reference of this issue in the other [root] ticket. Tag #823

[yogeshgadge]

@amcdnl Will this fix the space that we get when used inside mat-select ?

<mat-form-field>
	<mat-select placeholder="State">
		<cdk-virtual-scroll-viewport [itemSize]="states.length">
			<mat-option *cdkVirtualFor="let state of states"
                                             [value]="state">
                              {{state}}
                         </mat-option>
		</cdk-virtual-scroll-viewport>
	</mat-select>
</mat-form-field>

https://stackblitz.com/edit/angular-h4xptu-dgjd87?file=app/select-reset-example.html ?

[CharlBest]

@mmalerba it seems development on the autosize strategy had slowed/stopped. Is this because @amcdnl is on holiday/left for a while or are there other reasons for it?

And if it is because the team is just busy with other stuff that's fine just wanted to know. :)

[mmalerba]

I'm the one primarily working on it, but I've been busy with other things lately. I'll have some time to work on it more after the 7.0 release

[CharlBest]

Ok thanks for the update. Appreciate it. Good luck for the major release.

[ngbot]

Hi @amcdnl! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

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