RFR: JDK-8255439: System Tray icons get corrupted when windows scaling changes [v4]

[Harshitha Onkar]

On Tue, 10 May 2022 23:17:36 GMT, Harshitha Onkar <duke at openjdk.java.net> wrote:

>> In Windows, when desktop scaling is changed the tray icons was distorted/blurred a bit each time scaling changes.
>> 
>> With the proposed fix, the tray icon scales according to on-the-fly DPI scale settings. A test case has been added which adds a MRI icon to system tray, to observe the icon scaling when DPI is changed. Since the scale cannot be programmatically changed (for dynamic on-the-fly scale changes), I have used a manual test case to test this scenario.
>> 
>> When DPI changes usually two messages are sent by windows -
>> 
>> - [WM_DPICHANGED](https://docs.microsoft.com/en-us/windows/win32/hidpi/wm-dpichanged)
>> - [WMPOSCHANGING](https://docs.microsoft.com/en-us/windows/win32/winmsg/wm-windowposchanging)
>> 
>> I'm triggering an update on tray icons on receiving WMPOSCHANGING msg through the Tray icon's Window Procedure. Triggering an update on WM_DPICHANGED was still causing the icons to be distorted, hence WMPOSCHANGING is being used as the message to trigger the update.
>
> Harshitha Onkar has updated the pull request incrementally with one additional commit since the last revision:
> 
>   test case exit issue fix

I think the icon distortion observed on Win10 and 11 might be due to race condition between update triggered by WM_WINDOWPOSCHANGING and the re-addition of tray icons when taskbar is reloaded (occurs when DPI changes)

Whenever DPI changes, the taskbar is reloaded and all the existing tray icons are re-added by `WmTaskbarCreated()` in `awt_TrayIcon.cpp`. Here is the link to the relevant doc: [Taskbar creation and reload](https://docs.microsoft.com/en-us/windows/win32/shell/taskbar#taskbar-creation-notification) 

**The reason for distortion:** 
When update to tray icons (creating new scaled icon and deleting the old one) is called on `WM_WINDOWPOSCHANGING`, it might create race condition - updating the tray icon and deleting the previous ones while the  WmTaskbarCreated is trying to re-add the icons at the same time.

To overcome the race condition, I have removed the call to **UpdateTrayIconHandler()**  on WM_WINDOWPOSCHANGING and added it when taskbar gets reloaded and before `WmTaskbarCreated` is called.

When tested on Win10, it works as well. 

@aivanov-jdk Can you please test on Win10 & 11 with the new fix and let me know if it works?

-------------

PR: https://git.openjdk.java.net/jdk/pull/8441