Inserir UpdateStyles
logo após mover a janela para o topo é o truque:
AddToFunc MyUpdate
+ I WindowStyle StaysOnTop
+ I UpdateStyles
+ I WindowStyle NoTitle
Da documentação:
Delayed Execution of Commands
Note: There are many commands that affect look and feel of specific, some or all windows, like Style, Mouse, Colorset, TitleStyle and many others. For performance reasons such changes are not applied immediately but only when fvwm is idle, i.e. no user interaction or module input is pending. Specifically, new Style options that are set in a function are not applied until after the function has completed. This can sometimes lead to unwanted effects.
To force that all pending changes are applied immediately, use the UpdateStyles, Refresh or RefreshWindow commands.