RFR: 8359899: Stage.isFocused() returns invalid value when Stage fails to receive focus

[Lukasz Kostyra]

On Wed, 16 Jul 2025 08:50:31 GMT, Lukasz Kostyra <lkostyra at openjdk.org> wrote:

> This PR fixes `isFocused()` returning invalid value when Stage fails to receive focus after calling `Stage.show()`. This problem is Windows-only.
> 
> In Windows the `SetForegroundWindow()` API lists [a set of conditions to successfully grant focus to a Window](https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-setforegroundwindow#remarks). If any of the conditions are not met, the API will return FALSE. JavaFX did not respect that and instead assumed that receiving `WM_ACTIVATE` with our Window being activated is enough to assume the Window is in focus (which in some cases is not true).
> 
> I first tried reacting to `WM_SETFOCUS` and `WM_KILLFOCUS` but it seems those messages are not sent when the window is shown for the first time (instead `WM_ACTIVATE` is used).
> 
> To correct this behavior, I noticed the following path is the most reliable:
> - Call `ShowWindow()` using `SW_SHOWNA` instead of `SW_SHOW` - that makes the window visible but does NOT activate it
> - Call `SetForegroundWindow()` - that will attempt to give the Window focus and will also activate it if it is successful
>   - If successful, Java `notifyFocus` callback will be called via `WM_ACTIVATE` handler
>   - If it fails, we call the `notifyFocus` callback manually informing the upper layers the focus is lost. This establishes the correct state of `Window.focused` property.
> 
> With this change I observed that all tests pass as intended as long as two conditions are met (these are needed to satisfy `SetForegroundWindow()` restrictions):
> - Gradle build is ran without the Gradle daemon
> - The terminal running Gradle test is in foreground
> 
> If any of above two conditions is not met, some tests (including canary test from https://github.com/openjdk/jfx/pull/1804) now timeout/fail when checking whether `Window.isFocused()` is true.
> 
> Manually started JavaFX apps (ex. Ensemble) run as they used to and still receive focus upon Stage showing.

I didn't get a chance to look into other solutions yet, but I will be investigating that soon.

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PR Comment: https://git.openjdk.org/jfx/pull/1849#issuecomment-3436732981