Virtualization issue when grouping enabled with CacheLength=0

[CodingOctocat]

When CacheLength is set to 0 and grouping is enabled, virtualization does not behave correctly. It appears that items are not realized/measured as expected during scrolling.

From what I can observe, the issue seems related to how the cache area is calculated. Specifically, it looks like the measurement of the cache region might be ignoring the height of group headers.

VirtualizingPanel.CacheLength="0"
VirtualizingPanel.IsVirtualizingWhenGrouping="True"

vwp-issue-260402.mp4

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VirtualizingPanel.CacheLength="1"
VirtualizingPanel.IsVirtualizingWhenGrouping="True"

As long as CacheLength is greater than 0, at least there won't be any issues with the visual experience.

vwp-issue-260402-ok.mp4