Whenever you winset something through JS, it doesn't work. It used to work on 515.
{"<div id = "buttonsholder"><span class="button"><a class="buttonlink" href="byond://winset?id=[id]&is-visible=false">❌</A></span></div>"}
This line of code made me initially check it.
Testcase: https://www.dropbox.com/scl/fi/oizzsghxfxn338f5654y3/ winsetbug.zip?rlkey=2wpp551v0i656xjcjcie5rg69&st=7go4g7zy&dl =0