[release/11.0-rc1] Fix InitialItemIndex viewport underfill for small items in big container or on window resize

[github-actions]

Backport of #67936 to release/11.0-rc1

/cc @ilonatommy

Fix  InitialItemIndex viewport underfill for small items in big container or on window resize

Loading the list with first item different than ID=0 we were supressing callbacks until the top of the requested item was aligned with the top of the viewport. In cases when one callback was not enough to cover the whole viewport, part of the viewport stayed always blank (due to suppressed callbacks).

Description

JS is checking if blank area exists: it is most frequently at the bottom but in cases when list is being loaded with initial item index close to the highest available index, the blank space might be at the top (or both at top and bottom at the same time). JS informs C# about the viewport coverage by returning ViewportFillDirection from AlignToTargetAsync request. C# acts according to that instruction: Covered - we're done, InitialIndexState is completed, Before - upper spacer should load data (it tries to load twice as much as the number of already loaded items), After - bottom spacer should load data. We never (accumulatively) load more than MaxItemCount + 2 * OverscanCount, where OverscanCount are items above and below the viewport, by default 15.

Fixes #67936

Customer Impact

Before user had to start scrolling to recover from "partially blank" viewport. Now the full viewport is covered.

Regression?

• Yes
• No

InitialItemIndex is a net11 feature.

Risk

• High
• Medium
• Low

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A