fix: Ensure the show desktop line is always 1 physical pixel regardless of system scaling

as title

Log: as title
Pms: BUG-302035

Author: yixinshark

panels/dock/showdesktop/package/showdesktop.qml

@@ -4,6 +4,7 @@
 
 import QtQuick 2.15
 import QtQuick.Controls 2.15
+import QtQuick.Window 2.15
 
 import org.deepin.ds 1.0
 import org.deepin.dtk 1.0 as D
@@ -32,8 +33,10 @@ AppletItem {
 
         Rectangle {
             property D.Palette lineColor: DockPalette.showDesktopLineColor
-            implicitWidth: useColumnLayout ? showdesktop.implicitWidth : 1
-            implicitHeight: useColumnLayout ? 1 : showdesktop.implicitHeight
+            // Use device pixel ratio to ensure the line is always 1 physical pixel regardless of system scaling
+            property real devicePixelRatio: Screen.devicePixelRatio
+            implicitWidth: showdesktop.useColumnLayout ? showdesktop.implicitWidth : (1 / devicePixelRatio)
+            implicitHeight: showdesktop.useColumnLayout ? (1 / devicePixelRatio) : showdesktop.implicitHeight
 
             color: D.ColorSelector.lineColor
         }