I know it's been a year, but I'm not sure where else would be best to ask this, so:
Are there plans to support XDG_DECORATION in WINE ~11, and will it prefer SSDs on the application-side?
Aside from that, for those who have to deal with the CSD titlebars, is there any desire to upgrade the CSDs from Windows Classic to Windows Basic CSDs? All the textures for Windows Basic exist in Windows XP msstyles, afterall, given Windows Basic derivatives from Windows XP's window manager afterall, and this solution would also ensure consistency with programs that use Windows Basic's textures such as Mozilla Firefox.
Upgrading the CSDs to Windows Basic, especially now they're exposed to users as a requirement in GNOME, would likely be the preferred way, for users of visual styles, to have the CSDs be improved, would still provide the opportunity for WINE to have its own custom window decorations due to the WINE Visual Style, and would also preserve the Windows Classic CSDs for users who opt out of Visual Styles entirely. If GNOME CSDs are desired later down the line, those could be supplied via the "Use GTK+ theme as Visual Style" experiment or new Wine Configuration option.