fix: prevent plugin iframe blank on Library re-render

[Brandon32]

Description

Fixes plugin iframe rendering blank when clicked immediately after opening the Library. The iframe src was being removed during re-render when library content loaded, leaving the plugin blank.

• Added _activePane state to track which plugin is currently active
• Used repeat() with keys to preserve iframe DOM during re-renders
• Set iframe src only when plugin becomes the active pane

Related Issue

• MWPW-186591
• Plugins iframe blank after Library re-render when opened immediately #719

Motivation and Context

When opening a plugin quickly after opening the Library (before all blocks/icons finish loading), the plugin iframe src attribute was removed during a re-render triggered by the library content load completion. This resulted in a blank iframe with cleared DOM.

How Has This Been Tested?

Local testing and browser override for da-bacom in the editor.
http://localhost:3000/edit#/aem-sandbox/block-collection/demo
https://plugin-render--da-live--brandon32.aem.page/

• Opening plugins immediately after Library opens and verified plugin loads correctly and iframe is not blank.
• Added unit tests for active pane state management and iframe src updates

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[aem-code-sync]

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