Off Canvas dialog background color bleeds through

cosmicdreams → changed the visibility of the branch 3485682-off-canvas-dialog to hidden.

cosmicdreams → changed the visibility of the branch 3485682-off-canvas-dialog to active.

Do you have a theme in mind that is particularly problematic (or a way I can fake it within a core theme)? I tested with stark and olivero as front-end theme and tried fiddly with the background on the body element within the iframe but couldn't get it to look bad.

No sorry. I mainly have seen this when I'm using the same page preview on projects. When a site builds a custom theme and they aren't establishing a base color, then the off-canvas dialog's dark color make an outsized impact on how the preview appears.

I think, if you were to hack olivero to remove all of it's background color from body / html / * you would see it.

I have the issue and it seems that the issue has something to do with the body background colour not being defined. This means that body is transparent and actually relies on the browser/os setting the correct default colour.

When the body content and styles are rendered on top of another element that has set a background colour it shows through.

The preview should use as the highest level content content (or it's parent):
background-color: Window;

See: https://www.w3.org/TR/css-color-3/

@simohell I'll have to manually test this. The background color of "Canvas" appears to be defaulted to a dark color. I worried this would be continuance of the issue I'm seeing where this color makes it difficult to see dark text when the body doesn't define a background color. The choice of white was to give a color similar to the Body's default color.

Canvas is a system color so it varies. On my site and my browser it is white. It should be same colour as html file with no css or color attributes. With CSS 2 it would have been "Window" but that is deprecated in CSS 3, I think.

This change is straightforward in code and fixes the issue on an actual site that has the problem with background color.

Automatically closed - issue fixed for 2 weeks with no activity.