[Reg 5.15.0 -> 5.15.1] header's width isn't resized to window's width when Layout is used

header's and footer's width should be resized to Window's width according to https://doc.qt.io/qt-5/qml-qtquick-controls2-applicationwindow.html#header-prop
However, it is not resized in Qt5.15.1 when layout is used. This works fine with Qt5.15.0.

• The problem appears only when the app starts. If you resize the window, the header's width is adjusted accordingly.
• The header's width is adjusted properly when using other component, e.g. Rectangle

The workaround would be to bind the item's width to the parent.width but this shouldn't be necessary.

ApplicationWindow {

    header: RowLayout {
        Label {
            text: "Label 1"
        }
        Label {
            text: "Label 2"
        }
        Label {
            text: "Label 3"
        }
    }

How to reproduce :

• Use Qt5.15.1
• Run QTBUG-87708_ApplicationWindowResizeIssue.zip

Expected result (Also the result of Qt5.15.0)

Actual result