This appears to be fixed on KDE Neon Testing.  Can you check if it is fixed for you as well?

That seems like it might be a Qt 6.7 specific issue, I also have git master stuff but with Qt 6.6.1 and the dialog looks fine for me.

Also looks fine for me with Qt 6.6.0.

Indeed, it's a change from Qt 6.7. I'm not sure I want to call it a bug, the code is rather weird. See this minimal example:

import QtQuick
import QtQuick.Layouts
import QtQuick.Controls as QQC2

Window {
    id: root

    width: contentsControl.implicitWidth
    height: contentsControl.implicitHeight

    QQC2.Control {
        id: contentsControl

        contentItem: ColumnLayout {
            QQC2.Label {
                Layout.fillHeight: true
                text: "Something something A"
            }
            QQC2.CheckBox {
                checked: true
                text: "Something something B"
            }
        }

        anchors.fill: parent
        anchors.margins: 8
    }
}

With 6.7 the text and the checkbox slightly overlap each other, in 6.6 they don't

Nicolas, your example is a broken layout. It sets outer (window's) size to the control's implicit size, but the control is surrounded by additional margins, so it's not actually sized to implicit size but it slightly less instead. It can't possibly size itself correctly.

A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/4390