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

[lukostyra]

NOTE: For now I submitted this change as a DRAFT PR in order to get some feedback on whether this is the way to fix the problem, as well as to check for potential problems with this solution that I might've missed.

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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. 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 #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.

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Progress

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Issue

• JDK-8359899: Stage.isFocused() returns invalid value when Stage fails to receive focus (Bug - P3)

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$ git pull https://git.openjdk.org/jfx.git pull/1849/head

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$ git pr checkout 1849

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https://git.openjdk.org/jfx/pull/1849.diff

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