fix: GroupBox label background is always white

修正 GroupBox 标签文本的背景颜色总是白色的问题,会导致在暗色主题下
文字无法被看清楚.

注:只改了 qt 6 的版本.

Log:

Author: BLumia

examples/qml-inspect/Example_GroupBox.qml

@@ -0,0 +1,31 @@
+// SPDX-FileCopyrightText: 2025 UnionTech Software Technology Co., Ltd.
+//
+// SPDX-License-Identifier: LGPL-3.0-or-later
+
+import QtQuick 2.0
+import QtQuick.Layouts 1.11
+import org.deepin.dtk 1.0
+
+Flow {
+    id: control
+    Row {
+        spacing: 20
+
+        GroupBox {
+            title: "GroupBox"
+            ColumnLayout {
+                anchors.fill: parent
+                CheckBox { text: "E-mail" }
+                CheckBox { text: "Super Fancy Calendar" }
+                CheckBox { text: "Contacts" }
+            }
+        }
+
+        GroupBox {
+            title: "GroupBox 2"
+            Label {
+                text: "GroupBox Content"
+            }
+        }
+    }
+}

examples/qml-inspect/main.qml

@@ -140,6 +140,9 @@ D.ApplicationWindow {
         V2.TabButton {
             text: qsTr("Flickable")
         }
+        V2.TabButton {
+            text: qsTr("GroupBox")
+        }
     }
     StackLayout {
         anchors.left: parent.left
@@ -159,6 +162,7 @@ D.ApplicationWindow {
         Example_Menu {}
         Example_Popup {}
         Example_Flickable {}
+        Example_GroupBox {}
     }
 
     D.BehindWindowBlur {

examples/qml-inspect/qml.qrc

@@ -13,5 +13,6 @@
         <file>Example_Menu.qml</file>
         <file>Example_Popup.qml</file>
         <file>Example_Flickable.qml</file>
+        <file>Example_GroupBox.qml</file>
     </qresource>
 </RCC>

qt6/src/qml/GroupBox.qml

@@ -26,8 +26,6 @@ T.GroupBox {
         color: control.palette.windowText
         elide: Text.ElideRight
         verticalAlignment: Text.AlignLeft
-
-        background: Rectangle { }
     }
 
     background: Rectangle {