Custom HTML block preview keeps expanding when iframe uses height:100vh

[KeniVinh]

Description

I’m using the WordPress Custom HTML block to embed an iframe, for example:

<iframe src="https://www.photopea.com" style="width:100%; height:100vh; border:0;"></iframe>

The frontend displays normally, but in the block editor, the Custom HTML preview becomes very tall, shows a vertical scrollbar, and the editor becomes heavy. After saving/reopening the post, the block height seems to keep expanding.

Is there any way to disable the preview mode/review feature for the Custom HTML block, or prevent iframe content with height:100vh from being rendered in the editor preview?

Step-by-step reproduction instructions

1. Add a Custom HTML block.
2. Click Edit HTML.
3. Add an iframe in the HTML tab. The iframe uses height:100vh:

<iframe src="https://www.photopea.com" style="width:100%; height:100vh; border:0;"></iframe>

4. Click Update.
5. After updating, the Custom HTML block preview becomes very tall and shows a vertical scrollbar in the editor. The editor becomes heavy, even though the frontend page displays normally.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure

[hbhalodia]

Hi @KeniVinh, I have raised the PR with a fix. Could you please test the same and check if it resolves the issue related to infinte expansion and scroll.

Thanks,