I've enabled Dark Mode in Windows 10.
Yet, the native Firefox title bar does not support dark theme thus stays white.

Reproducible on latest nightly (2020-01-08)

Somewhat relevant: https://github.com/microsoft/terminal/blob/9dc922fc37a479dd560675ac4ea1ce3713ae0ada/src/interactivity/win32/windowtheme.cpp#L1-L118

The priority flag is not set for this bug.
:jimm, could you have a look please?

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How did you get the old titlebar to display? I don't have one in my browser. I wonder if this is a dev edition issue only.

Customize → Title Bar

Ok, we need to add support for this via the dwm apis. I managed to fix this in a test app using

BOOL flag = true;
DwmSetWindowAttribute(hWnd, 19, &flag, sizeof(BOOL));

19 is an undefined DWMWINDOWATTRIBUTE. That's probably going to show up in a future sdk.

Some testing needs to be done to understand when we should set this.

I'd be happy to mentor a contributor on this. Since this option is buried pretty deep in our options and likely not a popular setting, I see no reason to track.