2032664 - [Windows 11] Popup panels (bookmarks, app menu, extension panels) should support Mica/Acrylic backdrop effect

Status: NEW

(Firefox :: Theme, enhancement)

Description

[Zenon]

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:150.0) Gecko/20100101 Firefox/150.0

Steps to reproduce:

Steps to reproduce:

Enable Windows 11 Mica effect for Firefox (e.g., via browser.tabs.allow_transparent_browser or system settings)
Open any popup panel: Bookmarks menu, App menu (hamburger button), Extensions panel, or Settings dropdown

Actual results:

Only the main browser window/titlebar has the Mica effect. All XUL <panel> popups render with a solid opaque background, breaking visual consistency.

Expected results:

Popup panels should have the same Mica/Acrylic transparent backdrop effect as the main browser window, consistent with Windows 11 design language.

Additional context:

Firefox already supports Mica for the main window via DwmSetWindowAttribute(DWMWA_SYSTEMBACKDROP_TYPE) on the top-level nsWindow.
However, popup widgets (nsWindow with ePopup type) do not call DwmSetWindowAttribute to enable Mica/Acrylic.
The Zen Browser (a Firefox fork) has successfully implemented this by applying DWMWA_SYSTEMBACKDROP_TYPE to popup nsWindow instances as well, proving it is technically feasible.
Relevant source: widget/windows/nsWindow.cpp — the UpdateDwmAttributes() method could be extended to handle popup windows.