How to work around no "layout: blank" option in Confluence modules (manifest.yml), like in Jira

Note: We’re using custom UI.

Jira’s page modules (e.g., https://developer.atlassian.com/platform/forge/manifest-reference/modules/jira-project-page/#properties) have a layout: blank option that removes a built-in header and allows the iframe to take up the whole panel.

However, in Confluence’s page modules (e.g., https://developer.atlassian.com/platform/forge/manifest-reference/modules/confluence-space-page/#properties), no such option exists.

Here’s the unhideable built-in header in Confluence:

The extra header is unnecessary… we just want to have our own header rather than have the less attractive built-in one.

And here’s what happens when we try to show a modal in the iframe:

It doesn’t take up the whole panel, so it doesn’t look like a real modal.

Has anyone found a workaround for this issue? Is there another manifest.yml setting I’m missing besides layout: blank in Confluence? Or another trick?