RFR: 8311689: Wrong visible amount in Adjustable of ScrollPane [v2]

[Alexey Ivanov]

> It proved that `ScrollPane.layout` depends on the result of `WScrollPanePeer.childResized`, specifically on `setSpans` which recalculates the spans and sets the insets.
> 
> https://github.com/openjdk/jdk/blob/b285ed72aebe2d802fa9c071372cea6c09870b9a/src/java.desktop/share/classes/java/awt/ScrollPane.java#L514-L521
> 
> After the fix for [JDK-8297923](https://bugs.openjdk.org/browse/JDK-8297923), `setSpans` that is called in `childResized` is run asynchronously on the toolkit thread. Therefore `getViewportSize` uses the wrong insets which don't take into account the size of the scroll bar. Because of it, the `visibleAmount` field of adjustables is also wrong, and the scroll pane cannot display a portion of its child component.
> 
> I overlooked this dependency even when I was fixing the first regression, [JDK-8310054](https://bugs.openjdk.org/browse/JDK-8310054). Had I followed Harshitha's advice in #14478, I would've fixed this problem too. Similarly, Phil's intuition was right: `setSpans` should be synchronous. @honkar-jdk @prrace
> 
> **Fix**
> 
> The fix is to run `AwtScrollPane::_SetSpans` synchronously using `AwtToolkit::InvokeFunction`.
> 
> I also addressed [Sergey's concern](https://github.com/openjdk/jdk/pull/14478#discussion_r1239049983): `VerifyState` is now called after `SetInsets`. @mrserb
> 
> Client tests are green.

Alexey Ivanov has updated the pull request incrementally with one additional commit since the last revision:

  Add Robot.waitForIdle
  
  It ensures getViewportSize in the test is called
  after AwtScrollPane::_SetSpans completes on the
  toolkit thread.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14815/files
  - new: https://git.openjdk.org/jdk/pull/14815/files/f9e59211..f40921ca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14815&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14815&range=00-01

  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14815.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14815/head:pull/14815

PR: https://git.openjdk.org/jdk/pull/14815