RFR: 8297923: java.awt.ScrollPane broken after multiple scroll up/down

[Alexey Ivanov]

On Tue, 6 Jun 2023 16:07:53 GMT, Alexey Ivanov <aivanov at openjdk.org> wrote:

> **Problem description**
> 
> If you grab the thumb of the scroll bar of `ScrollPane` and drag it slowly and continuously up and down, you'll notice the UI stops rendering correctly: the child component of the scroll pane will render on the left of the frame itself, the inside of the scroll pane will be filled with the background color of its child component.
> 
> **Root cause**
> 
> AWT calls the [`::SetScrollInfo`](https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-setscrollinfo) function on EDT when AWT processes scroll bar tracking events. This Windows API is not thread-safe, calling this function on an incorrect thread leads to leaking GDI objects.
> 
> When the process reaches the limit on the number of GDI objects, it cannot create new GDI objects, which results in rendering issues.
> 
> **Fix**
> 
> To resolve the problem, I modified the code so that `::SetScrollInfo` and [`::GetScrollInfo`](https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getscrollinfo) are always called on the toolkit thread, all AWT components are created on the toolkit thread.
> 
> An automatic test is provided. The test scrolls the vertical scroll bar up and down. Then the test takes a screenshot. When the bug is reproduced, the robot cannot create new GDI objects to capture the screenshot and it throws `OutOfMemoryError`.

The screenshot of the test case after the problem is reproduced:

![ScrollPaneTest](https://github.com/openjdk/jdk/assets/70774172/acdaa2e3-b123-448b-8145-47335c7f4d39)

If you scroll up or down, the left side of the frame gets filled with the light-green color of the canvas which is the child component inside the scroll pane.

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PR Comment: https://git.openjdk.org/jdk/pull/14338#issuecomment-1579061086