Fullscreen - z-index can be broken if editor is destroyed while in fullscreen mode and created again

[lkszzajac]

📝 Provide detailed reproduction steps (if any)

1. Open the manual test for the Fullscreen plugin.
2. Select the "custom container" checkbox and recreate the editor.
3. Enter fullscreen mode (in the custom container). Verify that dialogs/balloons like Link form display correctly.
4. Recreate the editor again and enter fullscreen mode again.
5. Open any dialog (e.g. Upload image from URL)

✔️ Expected result

Dialog displays as usual, above the fullscreen editor

❌ Actual result

Dialog displays below the editor.

Screen.Recording.2025-03-17.at.17.17.20.mov

📃 Other details

• Browser: n/a
• OS: n/a
• First affected CKEditor version: ck/epic/1235-fullscreen-mode
• Installed CKEditor plugins: Fullscreen

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[Dumluregn]

Fixed in #18191.