Don't reset xfader in const power mode when disabling crossfader.

This fixes https://bugs.launchpad.net/mixxx/+bug/1967941

src/library/autodj/autodjprocessor.cpp

@@ -3,6 +3,7 @@
 #include "control/controlproxy.h"
 #include "control/controlpushbutton.h"
 #include "engine/engine.h"
+#include "engine/enginexfader.h"
 #include "library/trackcollection.h"
 #include "mixer/basetrackplayer.h"
 #include "mixer/playermanager.h"
@@ -166,6 +167,7 @@ AutoDJProcessor::AutoDJProcessor(
 
     m_pCOCrossfader = new ControlProxy("[Master]", "crossfader");
     m_pCOCrossfaderReverse = new ControlProxy("[Mixer Profile]", "xFaderReverse");
+    m_pCOCrossfaderMode = new ControlProxy("[Mixer Profile]", "xFaderMode");
 
     QString str_autoDjTransition = m_pConfig->getValueString(
             ConfigKey(kConfigKey, kTransitionPreferenceName));
@@ -184,6 +186,7 @@ AutoDJProcessor::~AutoDJProcessor() {
     m_decks.clear();
     delete m_pCOCrossfader;
     delete m_pCOCrossfaderReverse;
+    delete m_pCOCrossfaderMode;
 
     delete m_pSkipNext;
     delete m_pShufflePlaylist;
@@ -553,7 +556,13 @@ AutoDJProcessor::AutoDJError AutoDJProcessor::toggleAutoDJ(bool enable) {
                 &AutoDJProcessor::crossfaderChanged);
         deck1->disconnect(this);
         deck2->disconnect(this);
-        m_pCOCrossfader->set(0);
+
+        if (m_pCOCrossfaderMode->get() == MIXXX_XFADER_ADDITIVE) {
+            // Reset the crossfader for DJs using the line faders
+            // This works only without an audible effect in additive mode where
+            // the center is unity gain for both decks https://bugs.launchpad.net/mixxx/+bug/1967941
+            m_pCOCrossfader->set(0);
+        }
         emitAutoDJStateChanged(m_eState);
     }
     return ADJ_OK;

src/library/autodj/autodjprocessor.h

@@ -288,6 +288,7 @@ class AutoDJProcessor : public QObject {
 
     ControlProxy* m_pCOCrossfader;
     ControlProxy* m_pCOCrossfaderReverse;
+    ControlProxy* m_pCOCrossfaderMode;
 
     ControlPushButton* m_pSkipNext;
     ControlPushButton* m_pFadeNow;