RFR: 8346281: [Windows] RenderScale doesn't update to HiDPI changes

[Jose Pereda]

On Mon, 23 Dec 2024 10:51:39 GMT, Jose Pereda <jpereda at openjdk.org> wrote:

> This PR adds the missing native implementation for Windows `GlassWindow::HandleDPIEvent`, to notify the (Java) window when there is a DPI change event, which can happen when the user changes the resolution of the screen (via Settings -> System -> Display -> scale), while the JavaFX application is running.
> 
> When such `WM_DPICHANGED` event happens, `GlassWindow::HandleDPIEvent` notifies the (Java) window, which changes its platform scale via `Window::notifyScaleChanged`, and `GlassScreen::HandleDisplayChange();` is needed too, to update the platform scale of the screen where the window is at as well.
> 
> There are no tests added to this PR, since these would require manual intervention to change the display. In any case, the test case added to the [issue](https://bugs.openjdk.org/browse/JDK-8346281) runs fine now when the app runs on a given screen and the user changes its resolution. 
> 
> There is a follow-up issue after this PR, that might require a more complex fix, for the case where the user changes the resolution of a different screen that is placed before the one that has the application (as location coordinates of the latter depend on the former), because there is no `WM_DPICHANGED` event in this case.

I've added the test case to the JBS issue (sorry about that, I thought I did when I created it).

About the test case scenario:
- you have a JavaFX application already running in a given monitor
- without changing resolution, the label shows the current output scale, and the menu popup shows up in the correct position (if it doesn't, this is a different issue than this one).
- Then, after showing the menu at least once, you change the screen resolution (the screen that has the app, that is)

With this PR you should see: 
- the label showing the correct output scale once you change the resolution (this doesn't happen without the PR),
- and the menu popup showing up at the correct position (this doesn't happen without the PR).

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PR Comment: https://git.openjdk.org/jfx/pull/1668#issuecomment-2560305657