I have a situation where the window border for a layout is different, adding an additional border when in List view vs form or table view. Here are the examples (same layout).
View as Form:
View as List:
You will note the list view has a light gray border added on the left in the header part, and a black border added on the left in the body part.
I think this is a vestige of migrating this layout from prior versions of FileMaker, when "embossed" and "engraved" styles were allowed. I've painstakingly made my own theme and got rid of all those styles several versions ago, but something appears to be hidden deep in the CSS that is still showing this.
Interestingly, I can create a new layout from a default FileMaker theme (e.g. Enlightened), change the theme to my custom theme, and the problem does not exist. However, this would take too much time to create each layout, setup all the parts and copy all of the objects, then apply the specific theme style to each object for this to be viable given the number of layouts I have. Duplicating an existing layout, applying a default FileMaker theme, then reapplying my custom theme still has the problem, which leads me to believe it's a leftover bug in the layout not the theme.