Fix active cell displaying above row header cell

[catfo435]

Fixes #3380

Fixes the issue by changing the z-index value of the top-left-cell from 3 to 5.

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[seancolsen]

@catfo435, I just wanted you to know that I'm still planning to review this PR but that work is queued behind several higher priority tasks at the moment. I can't promise when I'll be able to review this, but I'll definitely get to it.

[seancolsen]

Note to self for review: PRs #3393 and #3419 coincidentally proposed exactly this same fix.

[seancolsen]

Thanks @catfo435

[catfo435]

No problem! @seancolsen