Graphic glitch when changing layoutDirection

idkfa

I have a qml project for MacOS. There is a frameless window where I have a big rectangle (mainView) with main functions and a small rectangle (expandRect) on a side which is used to drag the window so it can stick to one side of the screen or another.
I need the main rectangle always to stick to the screen side so I'm changing mainLayout.layoutDirection.

import QtQuick
import QtQuick.Controls
import QtQuick.Layouts
import QtQuick.Window

ApplicationWindow {
   id: browserWindow

   enum Side {
       Left,
       Right
   }

   property int currentSide: Main.Side.Right

   width: 432
   height: screen.desktopAvailableHeight
   visible: true
   flags: Qt.Window | Qt.FramelessWindowHint

   color: "transparent"

   Component.onCompleted: {
       stickToRight()
   }

   RowLayout {
       id: mainLayout
       anchors.fill: parent

       Rectangle {
           id: expandRect
           Layout.preferredWidth: 40
           Layout.preferredHeight: width
           radius: 7
           y: (parent.height - height) * 0.25

           MouseArea {
               anchors.fill: parent

               DragHandler {
                   onActiveChanged: {
                       if (active && !browserWindow.minimized)
                           startSystemMove()
                       if (x > screen.desktopAvailableWidth / 2)
                           stickToRight()
                       else
                           stickToLeft()
                   }
               }
           }
       }

       Rectangle {
           id: mainView
           Layout.fillWidth: true
           Layout.fillHeight: true
           radius: 7
       }
   }

   onCurrentSideChanged: {
       if (currentSide === Main.Side.Right) {
           console.log("Side changed to right")
           mainLayout.layoutDirection = Qt.LeftToRight
       }
       if (currentSide === Main.Side.Left) {
           console.log("Side changed to left")
           mainLayout.layoutDirection = Qt.RightToLeft

           browserWindow.width--
           browserWindow.width++
       }
       // to get rid of transparent main window frame on MacOS we need to update window like this
       // browserWindow.width--
       // browserWindow.width++
   }

   function stickToRight() {
       browserWindow.x = screen.desktopAvailableWidth - browserWindow.width
       browserWindow.y = 0
       currentSide = Main.Side.Right
   }

   function stickToLeft() {
       browserWindow.x = 0
       browserWindow.y = 0
       currentSide = Main.Side.Left
   }
}

When I move the window a few times, I get this strange frame border. I cannot even get what item is this.
Has someone even any tips on what can it be? If you set mainView.visible = false in "stick" functions you'll see that this is some kind of silhouettes of both rectangles. But what is it and how to get rid of them?
Any help is appreciated.