Review bar and "sticky" file header layout regression

[wxiaoguang]

Due to #26711 (and other old PRs) , now the "sticky" file header doesn't work well with uncertain review bar height.

Before, the sticky top was hard-coded to 2 magic numbers, but the "review bar" has been changed many times and it uses "flex" layout, the height is not predictable.

[silverwind]

Height needs to be predictable for sticky to work, unless there is another solution but I'm not aware of.

[wxiaoguang]

I am not working on it either, just open an issue for record.