If you start playing a video on youtube.com in Chrome, the browser creates a new button (☰♪) on its toolbar. If you click on the button, a popup appears that has a button “Enter picture-in-picture”. After clicking the button, Chrome opens a separate window with the video. Such a window has no borders.
FvwmIdent says it has NoClass & it’s not a transient window. If I try
Style NoClass Borders
the PIP window still stays borderless.
I also have
+ "Borders" Pick (CirculateHit) Style $[w.name] Borders + "No Borders" Pick (CirculateHit) Style $[w.name] !Borders
attached to a menu for a title button: the toggling works for every window, except for the PIP one.
Style * EWMHIgnoreStateHints
doesn’t help too.