fix(cdk/scrolling): prevent subpixel gaps in virtual scroll viewport

[Mr-KRAMA]

This PR fixes the issue where cdk-virtual-scroll-viewport renders visible gaps between items at certain zoom levels or scroll positions.

Problem:
Thin gaps (~0.5px) appear between consecutive rows when using *cdkVirtualFor

Issue occurs due to subpixel rounding in offset calculations

Most noticeable when zooming browser window or at specific scroll positions

Solution:
Added Math.round() to the offset calculation in setRenderedContentOffset()

Ensures all item positions are rounded to whole pixels

Eliminates subpixel gaps while maintaining smooth scrolling

Changes:

Single line change in fixed-size-virtual-scroll.ts

Wraps this._itemSize * newRange.start with Math.round()

Testing
Tested with different zoom levels (300%)

Verified no gaps appear between items during scrolling

Confirmed smooth scrolling behavior is preserved

Fixes #31824

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[mmalerba]

@Mr-KRAMA Can you remove the second commit? I think that failure was either a flake or an unrelated issue that can be fixed separately from this PR

[Mr-KRAMA]

@mmalerba Thanks for the feedback! I've removed the second commit as requested.

How should I approach the lighthouse audit failure? Should it be a separate PR or is it expected to be handled by maintainers?

[mmalerba]

This PR was merged into the repository. The changes were merged into the following branches:

• main: 2918e28
• 20.2.x: a9b1afd

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