[Widgets editor] TypeError: Cannot read property 'body' of null at onLoad

[adamziel]

Description

In some scenarios – and I am not sure which ones – the legacy widget preview results in an error TypeError: Cannot read property 'body' of null at onLoad. Stack trace points to the following line:

gutenberg/packages/widgets/src/blocks/legacy-widget/edit/preview.js

Line 110 in 253d9b6

event.target.contentDocument.body.style.overflow =

The problem occurs both in customizer and on widgets.php

Step-by-step reproduction instructions

1. Be a lucky owner of an affected site – I am not sure how to reproduce the pre-conditions, but there are no custom plugins or themes involved
2. Go to customizer or the widgets editor
3. Observe the error

Screenshots, screen recording, code snippet

No response

Environment info

No response

Pre-checks

• I have searched the existing issues.
• I have tested with all plugins deactivated except Gutenberg.

[adamziel]

There are two possibilities here:

1. event.target is somehow not an iframe
2. event.target is an iframe, but contentDocument is null

According to the HTML Living Standard:

The content document of a browsing context container container is the result of the following algorithm:

* If container's nested browsing context is null, then return null.
* Let context be container's nested browsing context.
* Let document be context's active document.
* If document's origin and container's node document's origin are not same origin-domain, then return null.
* Return document.

My guess is that the nested browsing context is not null because it was just populated, which leaves us with just one possibility: <iframe src="" /> points to a different origin than the document's origin. If so, this problem should be fixed by switching to <iframe srcdoc="" /> populated via apiFetch(): #34184

[noisysocks]

If so, this problem should be fixed by switching to <iframe srcdoc="" /> populated via apiFetch(): #34184

@adamziel @anton-vlasenko: Did we ever do this?

[noisysocks]

If so, this problem should be fixed by switching to <iframe srcdoc="" /> populated via apiFetch(): #34184

@adamziel @anton-vlasenko: Did we ever do this?

Looks like it's tracked in #33540..

[ndiego]

@noisysocks and @adamziel this issue came up in our weekly Editor Bug Scrub and I just wanted to check in. Seems like work on the Legacy Widget block has stalled? Just curious if this is an issue we are actively planning to fix, or if it can be closed as a "[Status] Not Implemented".

[noisysocks]

I don't believe anyone is working on this, no. If we're struggling to reproduce this and not hearing many reports from users then I'd be happy to close.

[ndiego]

Thanks for the follow-up @noisysocks. We reviewed it again during this week's Editor Bug Scrub, and the consensus was to close this ticket.