timeline: do not re-center the viewport vertically when the cursor move.

src/Gui/timeline/TracksView.cpp

@@ -1061,13 +1061,22 @@ TracksView::ensureCursorVisible()
 {
     if ( horizontalScrollBar()->isVisible() )
     {
-        QRectF r( m_cursorLine->boundingRect().width() / 2,
-                  m_cursorLine->boundingRect().height() / 2,
-                  1, 1 );
-        m_cursorLine->ensureVisible( r, 150, 50 );
+        QRectF visibleArea = visibleRect();
+        ensureVisible( cursorPos(),
+                       visibleArea.y() + ( visibleArea.height() / 2 ),
+                       1, 1, 150, 0 );
     }
 }
 
+QRectF
+TracksView::visibleRect()
+{
+    QPointF topLeft( horizontalScrollBar()->value(), verticalScrollBar()->value() );
+    QPointF bottomRight( topLeft + viewport()->rect().bottomRight() );
+    QMatrix reverted = matrix().inverted();
+    return reverted.mapRect( QRectF( topLeft, bottomRight ) );
+}
+
 void
 TracksView::updateDuration()
 {

src/Gui/timeline/TracksView.h

@@ -232,6 +232,11 @@ private slots:
      * \brief Ensure that the cursor is visible.
      */
     void                    ensureCursorVisible();
+
+    /**
+     * \brief Return the visible area of the viewport.
+     */
+    QRectF                  visibleRect();
     /**
      * \brief Update the global duration of the project.
      * This method should be called when an item is inserted, moved or removed to ensure