Fix pasting features to new layer doesn't paste fields

Fixes #17702

src/app/qgsclipboard.cpp

@@ -148,6 +148,12 @@ QString QgsClipboard::generateClipboardText() const
 
 void QgsClipboard::setSystemClipboard()
 {
+  // avoid overwriting internal clipboard - note that on Windows, the call to QClipboard::setText
+  // below doesn't immediately trigger QClipboard::dataChanged, and accordingly the call to
+  // systemClipboardChanged() is delayed. So by setting mIgnoreNextSystemClipboardChange we indicate
+  // that just the next call to systemClipboardChanged() should be ignored
+  mIgnoreNextSystemClipboardChange = true;
+
   QString textCopy = generateClipboardText();
 
   QClipboard *cb = QApplication::clipboard();
@@ -323,8 +329,22 @@ QByteArray QgsClipboard::data( const QString &mimeType ) const
   return QApplication::clipboard()->mimeData()->data( mimeType );
 }
 
+QgsFields QgsClipboard::fields() const
+{
+  if ( !mUseSystemClipboard )
+    return mFeatureFields;
+  else
+    return retrieveFields();
+}
+
 void QgsClipboard::systemClipboardChanged()
 {
+  if ( mIgnoreNextSystemClipboardChange )
+  {
+    mIgnoreNextSystemClipboardChange = false;
+    return;
+  }
+
   mUseSystemClipboard = true;
   emit changed();
 }

src/app/qgsclipboard.h

@@ -132,7 +132,7 @@ class APP_EXPORT QgsClipboard : public QObject
     /**
      * Source fields
      */
-    QgsFields fields() const { return !mUseSystemClipboard ? mFeatureFields : retrieveFields(); }
+    QgsFields fields() const;
 
   private slots:
 
@@ -180,6 +180,9 @@ class APP_EXPORT QgsClipboard : public QObject
     QgsCoordinateReferenceSystem mCRS;
     QPointer<QgsVectorLayer> mSrcLayer;
 
+    //! True if next system clipboard change should be ignored
+    bool mIgnoreNextSystemClipboardChange = false;
+
     //! True when the data from the system clipboard should be read
     bool mUseSystemClipboard = false;