Fix ComposePanel.requestFocus

[m-sasha]

ComposePanel.requestFocus does not currently work because the focus event delivered to contentComponent has a cause of UNKNOWN (or ACTIVATION, if called right when the window is shown for the first time), which is not handled.

This PR adds calling focusManager.moveFocus(FocusDirection.Enter) on these events.

Fixes JetBrains/compose-multiplatform#3888 (comment)

Testing

• Tested manually on the reproducer provided in the bug report.
• Added a unit test.

Also tested with

fun main() = SwingUtilities.invokeLater {
    val frame = JFrame()
    val button = JButton("Show Compose Panel")
    var composePanel: ComposePanel? = null

    fun addComposePanel() {
        val panel = ComposePanel()
        panel.setContent {
            var text by remember { mutableStateOf("") }
            TextField(
                value = text,
                onValueChange = { text = it },
            )
        }
        panel.size = Dimension(400, 200)
        panel.location = Point(0, 100)
        frame.contentPane.add(panel, BorderLayout.CENTER)
        composePanel = panel
    }

    button.addActionListener {
        if (composePanel == null)
            addComposePanel()
        else
            composePanel!!.isVisible = !composePanel!!.isVisible
        button.setText(if (composePanel!!.isVisible) "Hide ComposePanel" else "Show ComposePanel")
        if (composePanel!!.isVisible)
            composePanel!!.requestFocus()
    }
    frame.contentPane.add(button, BorderLayout.NORTH)
    frame.size = Dimension(400, 300)
    frame.isVisible = true
}

This could be tested by QA.

Release Notes

Fixes - Desktop

• Fixed ComposePanel.requestFocus(), making it correctly assign focus to the first focusable child