However, Fusion.ScrollBar looks good. Thank You.

Should I close the topic as resolved?

Sorry about the warnings, those are my fault. I said earlier that your scrollbars come from FluentWinUI3/Fusion, but on Windows the default is the native Windows style, and it can't be customized. That causes both messages, including the Unable to assign [undefined] to int on resize. Basing the custom scrollbar on Fusion's ScrollBar fixes it. Only these two lines change:

import QtQuick.Controls.Fusion as Fusion 

component FlatScrollBar: Fusion.ScrollBar {

Fusion is only used for this one component. All other controls keep the Windows style.

About the black rectangle on the scrollbar in 6.12 that isn't there in 6.8: if it shows up at 100% scaling too, or with Round, it looks like a regression. Could you file a bug report at https://bugreports.qt.io with your example and both screenshots (6.8 and 6.12)? Please post the link here so others can follow it.

qml test.qml
file:///C:/Qt/6.12.0/mingw64_162/bin/test.qml:12:24: QML QQuickRectangle: The current style does not support customization of this control (property: "contentItem" item: QQuickRectangle(0x21868a6e230, parent=0x0, geometry=0,0 4x4)). Please customize a non-native style (such as Basic, Fusion, Material, etc). For more information, see: https://doc.qt.io/qt-6/qtquickcontrols2-customize.html#customization-reference
file:///C:/Qt/6.12.0/mingw64_162/bin/test.qml:12:24: QML QQuickRectangle: The current style does not support customization of this control (property: "contentItem" item: QQuickRectangle(0x21868a6d6f0, parent=0x0, geometry=0,0 4x4)). Please customize a non-native style (such as Basic, Fusion, Material, etc). For more information, see: https://doc.qt.io/qt-6/qtquickcontrols2-customize.html#customization-reference
qml: 2

But with QT_SCALE_FACTOR_ROUNDING_POLICY=Round the problem is gone.
Qt 6.12.0 MinGW:

Qt 6.8.3 MSVC:

Also on every resizing it prints this message:

qrc:/qt-project.org/imports/QtQuick/Controls/Windows/ScrollBar.qml:37:49: Unable to assign [undefined] to int

I also see that in Qt 6.12 a black rectangle is drawn on the scrollbar, which is not present in Qt 6.8. But this problem is not related to this topic.

At 150%, one logical pixel is 1.5 physical pixels. Anything at an odd logical coordinate lands on half a physical pixel. Qt Quick doesn't snap item geometry to physical pixels, so a 1px line covers sometimes one and sometimes two physical pixels. If it's antialiased, it gets smeared across an extra pixel.

The scrollbars show this most. With the default style on Windows 11 (FluentWinUI3), the ScrollBar comes from Fusion: a thin, rounded and therefore antialiased handle. It changes while you resize because the window's logical size is a whole number but its physical size isn't always divisible by 1.5. For example, 515 physical pixels is 343.33 logical, but the window reports 343. Items attached to the right or bottom edge, like the scrollbars, end up half a physical pixel off in one direction or the other. Which way depends on the window width, so it flips with every pixel you resize.

I wrote earlier that the rounding policy should prevent this. I have to correct myself here. It's only true if it actually takes effect. With QT_SCALE_FACTOR_ROUNDING_POLICY=Round, 1.5 becomes 2, and at 2 the artifacts should disappear. Your log shows qml: 1.5, though. Could you set the variable in the same shell you start the app from and check what Screen.devicePixelRatio prints?

• If it prints 2 and the artifacts are still there, that's a bug and worth a report.
• If it still prints 1.5, the variable isn't reaching the application.

You can solve the problem by either bumping the scale factor to 2. That should solve everything, if 2 is acceptable.
Otherwise you can use a custom scrollbar, that is more forgiving. I experimented a bit:

import QtQuick
  import QtQuick.Controls
  
  Window {
      width: 343
      height: 100
      visible: true
      title: "DPR 1.5 Subpixel Artifact Test"
      
      component FlatScrollBar: ScrollBar {
          id: bar
          contentItem: Rectangle {
              implicitWidth: 4    // 4 logical = 6 physical pixels at 150%
              implicitHeight: 4   
              antialiasing: false 
              color: bar.pressed ? bar.palette.dark : bar.palette.mid
              opacity: 0.0
              
              states: State {
                  name: "active"
                  when: bar.policy === ScrollBar.AlwaysOn || (bar.active && bar.size < 1.0)
                  PropertyChanges { bar.contentItem.opacity: 0.75 }
              }   
              
              transitions: Transition {
                  from: "active"
                  SequentialAnimation {
                      PauseAnimation { duration: 450 }
                      NumberAnimation { target: bar.contentItem; duration: 200; property: "opacity"; to: 0.0 }
                  }   
              }   
          }   
      }   
      
      Rectangle {
          anchors.fill: parent
          color: "#f0f0f0"
          
          ScrollView {
              id: scrollView
              anchors.fill: parent
              
              ScrollBar.horizontal: FlatScrollBar {
                  parent: scrollView
                  x: scrollView.leftPadding
                  y: scrollView.height - height
                  width: scrollView.availableWidth
                  active: scrollView.ScrollBar.vertical.active
              }   
              
              ScrollBar.vertical: FlatScrollBar {
                  parent: scrollView
                  x: scrollView.mirrored ? 0 : scrollView.width - width
                  y: scrollView.topPadding 
                  height: scrollView.availableHeight
                  active: scrollView.ScrollBar.horizontal.active
              }   
              
              TextArea {
                  text: "Resize the window horizontally pixel by pixel to observe the 1px line / scrollbar shifting and thickening to 2px."
              }   
          }   
      }   
  }   

On Qt 6.8.3 MinGW 15.2 it starts with errors:
qml: 1.5
Failed to create swapchain. Retrying by requesting a software rasterizer, if applicable for the 3D API implementation.
Failed to build or resize swapchain
Failed to present: IDXGISwapChain is unavailable
Failed to end frame

If backend is OpenGL (set QSG_RHI_BACKEND=opengl) then window with any size starts without this problem, but the problem returns on window resizing.
Identical behavior on any Qt and compiler version.
Qt 6.8.3 (MSVC and MinGW):

Qt 6.12.0 MinGW:

Can you do me two more favors please:

• It's unlikely that the problem sits with MinGW. To exclude it completely, could you try MSVC instead?
• To see whether it's a regression, could you try an older Qt build, preferably 6.8?

Component.onCompleted:
    console.log(Screen.devicePixelRatio)

What's the output?