[Strict Mode] EuiMutationObserver doesn't work

[weronikaolejniczak]

Describe the bug

When Strict Mode is enabled, EuiMutationObserver doesn't work. Mutations are not being registered.

Docs page

Strict Mode	No Strict Mode

Impact and severity

It impacts all end-users of our consumers. Severe.

There is no known workaround.

[Search] EuiMutationObserver is used internally in:

• EuiDataGrid (Kibana, 71)
• EuiPopover (Kibana, 512)

[Search] EuiMutationObserver is used 2 times in Kibana

Environment and versions

• EUI version: ~
• React version: React 18 (Strict Mode)
• Kibana version (if applicable): ~
• Browser: ~
• Operating System: ~

Minimum reproducible sandbox

Codesandbox

To Reproduce

1. Enable Strict Mode.
2. Render EuiMutationObserver.
3. Notice that the changes observed are not actually registered and output.

Expected behavior

EuiMutationObserver works correctly and changes are registered and output correctly.