fix: Filter overlay in dashboard when scrolling

[michael-s-molina]

SUMMARY

Fixes #14828

@rusackas @junlincc We need to allocate time in the future to manage the z-index globally. The lack of a global index is creating these bugs because each developer assigns an isolated z-index value and it's difficult to track conflicts.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

119573651-6c3bb480-bd69-11eb-9268-536440f0ea17.mov

screen-recording-2021-06-13-at-122740-pm_2RyOMuas.mov

TESTING INSTRUCTIONS

See before/after videos for instructions.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #15140 (86d836e) into master (856a2bd) will not change coverage.
The diff coverage is n/a.

❗ Current head 86d836e differs from pull request most recent head c1b8344. Consider uploading reports for the commit c1b8344 to get more accurate results

@@           Coverage Diff           @@
##           master   #15140   +/-   ##
=======================================
  Coverage   77.49%   77.49%           
=======================================
  Files         969      969           
  Lines       49902    49902           
  Branches     6383     6383           
=======================================
  Hits        38671    38671           
  Misses      11029    11029           
  Partials      202      202           

Flag	Coverage Δ
javascript	72.37% <ø> (ø)

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Impacted Files	Coverage Δ
superset-frontend/src/components/Menu/Menu.tsx	67.05% <ø> (ø)
...d/components/DashboardBuilder/DashboardBuilder.tsx	91.02% <ø> (ø)

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[kgabryje]

LGTM