[8.18] [9.0] [Dashboard] Ensure resize handle does not overlap sticky header (#209937) (#210031)

# Backport

This will backport the following commits from `9.0` to `8.18`:
- [[9.0] [Dashboard] Ensure resize handle does not overlap sticky header
(#209937)](#209937)

<!--- Backport version: 9.6.4 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT [{"author":{"name":"Hannah
Mudge","email":"[email protected]"},"sourceCommit":{"committedDate":"2025-02-06T15:03:28Z","message":"[9.0]
[Dashboard] Ensure resize handle does not overlap sticky header
(#209937)\n\n## Summary\r\n\r\nSince
#207275 is too large
to\r\nbackport to `9.0`/`8.18` and will only be in `9.1`/`8.19`, I
wanted to\r\nat least backport **just** the z-index fix for the resize
handler
as\r\ndescribed\r\n[here](https://github.com/elastic/kibana/pull/207275/files#r1931305375)\r\n.
Unfortunately, the z-index used in that
PR\r\n(`euiTheme.levels.maskBelowHeader`) only works thanks to
other\r\nhover-action style changes, so I've had to set a hardcoded
z-index\r\n(`2000`) in this PR instead. This acts as a \"quick fix\" for
a pretty\r\nannoying bug while avoiding the risk of backporting the
entire\r\npresentation panel refactor PR to `9.0`/`8.18`\r\n\r\n\r\n|
Before | After |\r\n|--------|--------|\r\n| ![Screenshot 2025-01-23 at
5
49\r\n13 PM](https://github.com/user-attachments/assets/ea5f2ffe-46bd-4259-920a-8798a1a08775)\r\n|\r\n![image](https://github.com/user-attachments/assets/52a6ac63-5bb3-4cc0-a77d-99cac756aaa4)\r\n|","sha":"7ebe1bf8d020e5b732b22aa2a23adaae8bb5bb48","branchLabelMapping":{"^v9.0.0$":"main","^v8.18.0$":"8.x","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["Team:Presentation","loe:small","release_note:skip","impact:low","backport:version","v8.18.0"],"title":"[9.0]
[Dashboard] Ensure resize handle does not overlap sticky
header","number":209937,"url":"https://github.com/elastic/kibana/pull/209937","mergeCommit":{"message":"[9.0]
[Dashboard] Ensure resize handle does not overlap sticky header
(#209937)\n\n## Summary\r\n\r\nSince
#207275 is too large
to\r\nbackport to `9.0`/`8.18` and will only be in `9.1`/`8.19`, I
wanted to\r\nat least backport **just** the z-index fix for the resize
handler
as\r\ndescribed\r\n[here](https://github.com/elastic/kibana/pull/207275/files#r1931305375)\r\n.
Unfortunately, the z-index used in that
PR\r\n(`euiTheme.levels.maskBelowHeader`) only works thanks to
other\r\nhover-action style changes, so I've had to set a hardcoded
z-index\r\n(`2000`) in this PR instead. This acts as a \"quick fix\" for
a pretty\r\nannoying bug while avoiding the risk of backporting the
entire\r\npresentation panel refactor PR to `9.0`/`8.18`\r\n\r\n\r\n|
Before | After |\r\n|--------|--------|\r\n| ![Screenshot 2025-01-23 at
5
49\r\n13 PM](https://github.com/user-attachments/assets/ea5f2ffe-46bd-4259-920a-8798a1a08775)\r\n|\r\n![image](https://github.com/user-attachments/assets/52a6ac63-5bb3-4cc0-a77d-99cac756aaa4)\r\n|","sha":"7ebe1bf8d020e5b732b22aa2a23adaae8bb5bb48"}},"sourceBranch":"9.0","suggestedTargetBranches":["8.18"],"targetPullRequestStates":[{"branch":"8.x","label":"v8.18.0","branchLabelMappingKey":"^v8.18.0$","isSourceBranch":false,"state":"NOT_CREATED"}]}]
BACKPORT-->

Author: Heenawter

src/platform/plugins/shared/dashboard/public/dashboard_container/component/grid/use_layout_styles.tsx

@@ -50,7 +50,7 @@ export const useLayoutStyles = () => {
       }
 
       .kbnGridPanel--resizeHandle {
-        z-index: ${euiTheme.levels.mask};
+        z-index: 2000; // ensures the drag handle renders below sticky nav but abover panel contents on hover
 
         // applying mask via ::after allows for focus borders to show
         &:after {