Add an embedded block editor to the customizer - for widgets

[draganescu]

After the iterations and discussions in #26900 (Managing blocks in the Customizer), the idea exchange in the hallway hangout on the same subject, there resulted a few paths to explore:

• build a layer that allows the block editor to talk to the customizer (solves Can't edit or preview block widgets in the customizer #25818)
• Make it look like this:

Customizer.Block.Editor.for.Sidebars.mp4

These options went unexplored:

• [ ] attempting to embed a block's edit view in the preview and see what are the main technical issues we encounter
• [ ] improving the experience of using the editor in the sidebar with accessible resizing options
• [ ] improve the desktop experience of using the block editor in a constrained viewport size (starting with the block toolbar [Widgets] Fix toolbar not fixed to top and not horizontally scrollable. #25640)
• [ ] attempting a more fluid experience of using the editor in the sidebar with using the customizer's drawer UI (like the one for changing themes or adding classic widgets)

[celloexpressions]

For attempting to embed a block's edit view in the preview, transforming customize partials between the preview and the editor could happen at the sidebar (widget area) level. Clicking the edit icon transforms the customize partial to a frontend-embedded editor (within an iframe?). A similar icon, or navigating elsewhere, would transform the partial back to the preview with a selective refresh (using the current API that also triggers all hooks). Communication between the customize controls (top level) and preview (iframe) happens via postMessage.

This is a "modal" approach, but one that is inline and contextual. It keeps ample space for supplementary sidebar controls. Maybe the sidebar also allows quicker actions like reordering widgets without toggling into edit mode.

[shaunandrews]

Here's how I could see using the block editor to manage Widgets (well, Sidebars, really) within the Customizer:

Customizer.Block.Editor.for.Sidebars.mp4

[noisysocks]

I think we can close this. A lot of this exists in the plugin now and we are already using #24687 as an overview issue for the remaining bits and pieces.