[kbn-grid-layout] Disable pointer events on drag + resize (elastic#20…

…8647)

## Summary

This PR greatly improves the performance of `kbn-grid-layout` on drag +
resize by disabling pointer events during these actions. This is
especially evident when dragging over panels that have their own mouse
event listeners.

| Before | After |
|--------|--------|
|
![image](https://github.com/user-attachments/assets/efea4bd0-e950-4d71-8485-f5f123240e5b)
|
![image](https://github.com/user-attachments/assets/15b044b2-521c-4bf5-a6cb-673c3a2affe4)
|
|
![image](https://github.com/user-attachments/assets/7daa6c96-ff38-4336-beb8-e7378445e589)
|
![image](https://github.com/user-attachments/assets/37bb55c8-e757-4311-ab25-cb0c4cd18f58)
|
|
![image](https://github.com/user-attachments/assets/1fbe721d-690f-4a57-aaab-c07f575e5d41)
|
![image](https://github.com/user-attachments/assets/6bdfdd40-9e58-4750-b152-cec2b12065fc)
|

### Checklist

- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

src/platform/packages/private/kbn-grid-layout/grid/grid_height_smoother.tsx

@@ -33,14 +33,11 @@ export const GridHeightSmoother = ({
 
       if (!interactionEvent) {
         smoothHeightRef.current.style.minHeight = `${dimensions.height}px`;
-        smoothHeightRef.current.style.userSelect = 'auto';
         return;
       }
-
       smoothHeightRef.current.style.minHeight = `${
         smoothHeightRef.current.getBoundingClientRect().height
       }px`;
-      smoothHeightRef.current.style.userSelect = 'none';
     });
 
     return () => {

src/platform/packages/private/kbn-grid-layout/grid/grid_layout.tsx

@@ -161,6 +161,12 @@ export const GridLayout = ({
         css={css`
           padding: calc(var(--kbnGridGutterSize) * 1px);
 
+          // disable pointer events and user select on drag + resize
+          &:has(.kbnGridPanel--active) {
+            user-select: none;
+            pointer-events: none;
+          }
+
           &:has(.kbnGridPanel--expanded) {
             ${expandedPanelStyles}
           }