RFR: 8294426: Two fingers tap generates wrong mouse modifiers on M2 MacBooks

[Nikita Provotorov]

Hi there!
JetBrains has faced with a bug on Apple M2 MacBooks when tapping (_not_ pressing) with two fingers on a trackpad generates wrong mouse modifiers (which are returned by [MouseEvent.getModifiersEx](https://docs.oracle.com/en/java/javase/17/docs/api/java.desktop/java/awt/event/InputEvent.html#getModifiersEx())).

As far as I see, OpenJDK bug tracker still doesn't contain such a bug and I don't have rights to create a new one, so the PR doesn't refer any id. Here is the bug link from the JetBrains own tracker: [JBR-4765](https://youtrack.jetbrains.com/issue/JBR-4765/Cannot-invoke-context-menu-by-two-fingers-tapping-on-MacBook-with-M2-chip).

The bug is 100% reproducible on M2 MacBooks (at least under macOS 12.5). It's also reproducible on M1 MacBooks, but much more rarely (about 10-15% of reproducibility).

## Steps to reproduce
1. Enable `System Preferences` -> `Trackpad` -> `Tap to click`
2. Tap with two fingers in the following app:

import javax.swing.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;

class MouseWindow {
    final JFrame frame;

    MouseWindow() {
        frame = new JFrame();
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

        frame.setTitle("Mouse window");

        frame.addMouseListener(new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                System.out.println(e);
            }
        });

        frame.pack();

        frame.setSize(300, 300);

        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(MouseWindow::new);
    }
}


### Expected
Printed mouse event has `modifiersEx` is `4096` (which is `InputEvent.BUTTON3_DOWN_MASK`)

### Actual
Printed mouse event has `modifiersEx` is `4352` (which is `InputEvent.BUTTON3_DOWN_MASK | InputEvent.META_DOWN_MASK`)

## Evaluation
The following happens when a native mouse event reaches Java:
1. `NSEvent.nsToJavaModifiers(0)` called inside [CPlatformResponder.handleMouseEvent():81](https://github.com/openjdk/jdk/blob/5ae6bc23e857535532b59aae674e2b917bbf7284/src/java.desktop/macosx/classes/sun/lwawt/macosx/CPlatformResponder.java#L81) returns `0`. Earlier it always returned `4096` (`InputEvent.BUTTON3_DOWN_MASK`) but not in the cases of M2 tapping. For a trackpad press (not a tap) it still returns `4096`;
2. So, the `0` modifier comes into MouseEvent constructor which then [goes into MouseEvent.setOldModifiers()](https://github.com/openjdk/jdk/blob/5ae6bc23e857535532b59aae674e2b917bbf7284/src/java.desktop/share/classes/java/awt/event/MouseEvent.java#L800) which [initializes the field MouseEvent.modifiers to the value 4](https://github.com/openjdk/jdk/blob/5ae6bc23e857535532b59aae674e2b917bbf7284/src/java.desktop/share/classes/java/awt/event/MouseEvent.java#L1159) (it's `InputEvent.BUTTON3_MASK`);
3. Next, this constructed `MouseEvent` object is pushed into EDT queue.
4. Next, when a EDT thread pulls the event from the queue and starts to process it, it goes into `java.awt.LightweightDispatcher.retargetMouseEvent` and [creates a new MouseEvent based on the pulled one](https://github.com/openjdk/jdk/blob/5ae6bc23e857535532b59aae674e2b917bbf7284/src/java.desktop/share/classes/java/awt/Container.java#L4920) with the new value of the modifiers == `getModifiersEx() | getModifiers()`. From the p.2 we know, that `MouseEvent.modifiers` of the pulled event is `4`, so `getModifiersEx() | getModifiers()` is evaluated to `4`.
5. Next, the constructor of the new MouseEvent [goes into setNewModifiers()](https://github.com/openjdk/jdk/blob/5ae6bc23e857535532b59aae674e2b917bbf7284/src/java.desktop/share/classes/java/awt/event/MouseEvent.java#L795) (instead of `setOldModifiers` as in p.2) and initializes its own field modifiers to the value `4356` (`InputEvent.BUTTON3_MASK | InputEvent.BUTTON3_DOWN_MASK | InputEvent.META_DOWN_MASK`, see [setNewModifiers():1099](https://github.com/openjdk/jdk/blob/5ae6bc23e857535532b59aae674e2b917bbf7284/src/java.desktop/share/classes/java/awt/event/MouseEvent.java#L1099), [setNewModifiers():1129](https://github.com/openjdk/jdk/blob/5ae6bc23e857535532b59aae674e2b917bbf7284/src/java.desktop/share/classes/java/awt/event/MouseEvent.java#L1129)).
6. Thus, an app receives the instance of MouseEvent with `getModifiers()` == `4` and `getModifiersEx()` == `4352` so it thinks there is a `Command` keystroke.

## Fixing
Let's set manually inside `CPlatformResponder.handleMouseEvent` a mouse modifier which corresponds to the pressed button.

## What about a regression test
I've failed to write a regression test using Robot because it somehow forces the correct mouse modifiers (`NSEvent.nsToJavaModifiers()` correctly returns `InputEvent.BUTTON3_DOWN_MASK`), so I wrote a test that directly invokes `CPlatformResponder.handleMouseEvent` via reflection.

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

Commit messages:
 - 8294426: Two fingers tap generates wrong mouse modifiers on M2 MacBooks

Changes: https://git.openjdk.org/jdk/pull/10429/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10429&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294426
  Stats: 365 lines in 2 files changed: 365 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10429.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10429/head:pull/10429

PR: https://git.openjdk.org/jdk/pull/10429