Feature: Create a "linting" interface for errors, warnings, in the editor

[jasmussen]

Across a slew of issues, it has become clear that there are a lot of places content in a document you are authoring can have various levels of issues that need user attention. Things such as:

• Image block is empty
• Button/link is dead or empty
• Custom block is missing an API key
• Colors are missing contrast
• Navigation menu (cpt) has been deleted, but navigation block still refers to it
• Image is inserted in the editor but deleted in the media library
• Block is hidden
• Social Icons don't link anywhere
• Block has custom CSS
• Navigation items link to draft pages
• On-the-fly media sizes were interrupted during creation

These are just some of the different types of errors content in the editor can have. But common to them all: they refer to content errors that need user interaction.

#41747 detailed showing errors in the List View, but this would only catch local errors, and the List View is already crowded. In fact this very proposal for a new linting tool was instigated in part due to not wanting to over-burden the List View itself with additional labels and flags.

What I'd like to discuss—this is a proposal, but the design and concept needs feedback—is embracing the Notes tray as a visual linting tool. Shown here, with a notice-level lint item for indicating a block is hidden:

But the concept would go further: any notice, warning, or error, across the category of issues listed above, could potentially get a visual notice in context of a block. A thread, if a single block has multiple issues. It would provide big visual emphasis on items that need attention, and obvious flows to engage with them, as well as plenty of real-estate to be verbose with information.

Consider an OpenStreetMap block, unless engaged with, it could have a big Notice in the notes tray:

OpenStreetMaps needs an API key in order to function. [See instructions for getting your own key here], and then add it to the input field on the block itself.

[Mamaduka]

Maybe we can repurpose the Outline panel for this; the heading structure section already overlaps with it. We should also keep extensibility in mind when designing the API. This has been mentioned a couple of times for the Outline.

I can understand the direction to embrace the Notes tray, but since it's also in active development, overlapping features can make it more complicated.

P.S. I know pre- and post-public panels are sometimes used for similar cases. Do we plan to incorporate them?

[jasmussen]

That's a very valid option too. It could even start life in one place and move elsewhere, or be both—the more suggestion-like linting notes being more visual, other things in the Outline drawer. The main motivation for the in-canvas piece is the clear visibility, which we've heard loudly related to hidden blocks.

[adamsilverstein]

On-the-fly media sizes were interrupted during creation

This generally shouldn't happen in the editor - images are only added to the page when all sub-sizes have been successfully created.

That said, other things can lead to missing sub-sized images (such as switching themes), so I would just rephrase this to "Image block - some image sub-sizes are missing"