RFR: 8362091: Window title bar should reflect scene color scheme [v2]

[Michael Strauß]

On Sun, 27 Jul 2025 18:46:46 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> Currently, the color scheme of a system-decorated stage is as follows:
>> * On Windows, the title bar is always light (even if the OS color scheme is dark).
>> * On macOS and Linux, the title bar is light or dark depending on the OS color scheme.
>> 
>> The expected behavior is that the title bar matches the color scheme of the `Scene`.
>> If an application doesn't specify a color scheme, the title bar color should match the OS color scheme.
>> 
>> This PR fixes the behavior for Windows and macOS, but not for Linux (there's no good way to do that).
>> Depending on how you look at it, this is either a bug fix or an enhancement.
>
> Michael Strauß has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:
> 
>  - Merge branch 'master' into feature/dark-window-frame
>    
>    # Conflicts:
>    #	modules/javafx.graphics/src/main/native-glass/mac/GlassWindow.m
>  - Window decorations adapt to color scheme

> Is the extra code (beside looking at the version) so that I can toggle the darkbar from Java?

We have two places in JavaFX that are relevant to dark mode:
1. `Platform.Preferences.colorScheme`, which tells you the color scheme of the operating system (i.e. what you select in PC settings)
2. `Scene.Preferences.colorScheme`, which defaults to the OS color scheme, but is overridable for each scene.

This PR proposes to match the OS-provided title bar color to the value of `Scene.Preferences.colorScheme`, which allows developers to choose how their application should be presented:
* If you don't override the scene color scheme, the window title bar will match the OS by default (light or dark).
* However, if your application uses a fixed light or dark theme (i.e. it doesn't adapt its control theme colors to dark mode), then overriding the scene color scheme ensures consistent presentation independent of OS settings.

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PR Comment: https://git.openjdk.org/jfx/pull/1845#issuecomment-3218222926