Trying to remove popup 'Collapse' button in mobile layout

[FranklinAlexander]

Does anyone know how to remove the Collapse button in the popup header? The button doesn't do anything useful when in mobile layout and takes up valuable space. 

 I would like to remove the button altogether, but can't find a css solution. I have tried setting the display to none using the custom theme styling.ts file, but cannot find the right class/attribute/tag combination to get it to work. I was able to remove the Dock/Undock button using the following : 

calcite-action button[aria-label="Undock"] {

      display: none !important;

    }

Nothing has worked for the Collapse button, however, so I am wandering if it is even possible using the stylesheet. There may only be a coding solution to this, but I don't know where in the code the popup headers are being created. I know it's a longshot, but if anyone has a solution or suggestion that would be great. I am using EB Developer v1.14.