Fix autoExpand timer reset behavior (workaround for bug in Qt framework)

Starting with at least Qt 5.0.0 (released in 2011) and still present
in current versions of Qt (Qt 6.8.0 at the time of this commit), there
is a bug in the implementation of QTreeView::dragMoveEvent and autoExpandDelay:

QT BUG DESCRIPTION

Instead of resetting the delay timer whenever the mouse moves to a
new item, it is reset on every little mouse movement, which makes
autoExpand useless e.g. on laptop touchpads.

OUR WORKAROUND

Only reset the delay timer whenever the mouse has moved to a new item,
by bypassing QTreeView::dragMoveEvent() and directly calling
QAbstractItemView::dragMoveEvent() instead unless the mouse
has moved to a new item.

Author: cr7pt0gr4ph7

src/widget/wlibrarysidebar.cpp

@@ -83,6 +83,7 @@ void WLibrarySidebar::dragEnterEvent(QDragEnterEvent* pEvent) {
 /// or when the drag is aborted through Escape or other means.
 void WLibrarySidebar::dragLeaveEvent(QDragLeaveEvent* pEvent) {
     // qDebug() << "WLibrarySidebar::dragLeaveEvent";
+    m_autoExpandIndex = QModelIndex();
     toggleDragHoverPropertyAndUpdateStyle(false);
 
     QTreeView::dragLeaveEvent(pEvent);
@@ -101,22 +102,58 @@ void WLibrarySidebar::dragMoveEvent(QDragMoveEvent* pEvent) {
     // actual data being dragged is supported, e.g. whether it is
     // a list of valid track URLs.
     //
-    // Note: We go through QTreeView here instead of directly calling
-    // SidebarModel to retain other useful features from the base class,
-    // like e.g. auto-scroll behavior when the mouse cursor reaches
-    // the boundaries of the tree view.
+    // Note: We go through QTreeView/QAbstractItemView here, instead of
+    // directly calling SidebarModel, to retain other useful features
+    // from the base class, like e.g. auto-scroll behavior when the mouse
+    // cursor reaches the boundaries of the tree view.
     //
     // Note: pEvent->source() will be NULL if something is dropped
     // from a different application. This knowledge is used
     // inside the LibraryFeature implementations.
-    setSourceOfCurrentDragDropEvent(pEvent->source());
-    QTreeView::dragMoveEvent(pEvent);
-    setSourceOfCurrentDragDropEvent(nullptr);
+
+    // ========================================================================
+    // Fix autoExpand timer reset behavior (workaround for bug in Qt framework)
+    //
+    // Starting with at least Qt 5.0.0 (released in 2011) and still present
+    // in current versions of Qt (Qt 6.8.0 at the time of this commit), there
+    // is a bug in the implementation of QTreeView::dragMoveEvent and autoExpandDelay:
+    //
+    // QT BUG DESCRIPTION
+    //
+    // Instead of resetting the delay timer whenever the mouse moves to a
+    // new item, it is reset on every little mouse movement, which makes
+    // autoExpand useless e.g. on laptop touchpads.
+    //
+    // OUR WORKAROUND
+    //
+    // Only reset the delay timer whenever the mouse has moved to a new item,
+    // by bypassing QTreeView::dragMoveEvent() and directly calling
+    // QAbstractItemView::dragMoveEvent() instead unless the mouse
+    // has moved to a new item.
+    // ========================================================================
+    const QPoint pos = pEvent->position().toPoint();
+    const QModelIndex index = indexAt(pos);
+
+    if (m_autoExpandIndex != index) {
+        m_autoExpandIndex = index;
+        // QTreeView::dragMoveEvent just restarts the autoExpand timer
+        // and then calls QAbstractItemView::dragMoveEvent
+        setSourceOfCurrentDragDropEvent(pEvent->source());
+        QTreeView::dragMoveEvent(pEvent);
+        setSourceOfCurrentDragDropEvent(nullptr);
+    } else {
+        // Skip resetting the autoExpand timer (see above)
+        // because we are still hovering over the same item
+        setSourceOfCurrentDragDropEvent(pEvent->source());
+        QAbstractItemView::dragMoveEvent(pEvent);
+        setSourceOfCurrentDragDropEvent(nullptr);
+    }
 }
 
 // Drag-and-drop "drop" event. Occurs when something is dropped onto the track sources view
 void WLibrarySidebar::dropEvent(QDropEvent* pEvent) {
     // qDebug() << "WLibrarySidebar::dropEvent";
+    m_autoExpandIndex = QModelIndex();
     toggleDragHoverPropertyAndUpdateStyle(false);
 
     // QTreeView::dropEvent will, through some indirection, call

src/widget/wlibrarysidebar.h

@@ -53,4 +53,5 @@ class WLibrarySidebar : public QTreeView, public WBaseWidget {
     void setSourceOfCurrentDragDropEvent(QObject* pSource);
 
     int m_hoverExpandDelay;
+    QModelIndex m_autoExpandIndex;
 };