ui: Sumby drop down has the same z-index as other dropdowns

[manojVivek]

Screenshot:

[alwaysmeticulous]

✅ Meticulous spotted 0 visual differences across 585 screens tested: view results.

Meticulous evaluated ~4 hours of user flows against your PR.

_{Expected differences? Click here. Last updated for commit d5a1198. This comment will update as new commits are pushed.}