diff --git a/viewer/image_viewer/JFJochImage.cpp b/viewer/image_viewer/JFJochImage.cpp
index f1452e44..1f7a9479 100644
--- a/viewer/image_viewer/JFJochImage.cpp
+++ b/viewer/image_viewer/JFJochImage.cpp
@@ -64,6 +64,8 @@ JFJochImage::JFJochImage(QWidget *parent) : QGraphicsView(parent) {
 }
 
 void JFJochImage::onScroll(int value) {
+    if (suppress_overlay_update_)
+        return;
     updateOverlay();
 }
 
@@ -134,6 +136,10 @@ void JFJochImage::wheelEvent(QWheelEvent *event) {
         changeForeground(new_foreground);
         emit foregroundChanged(foreground);
     } else {
+        // Zooming and re-centering both move the scrollbars, and every move reaches
+        // onScroll(); suppress those and rebuild the overlay once, below.
+        suppress_overlay_update_ = true;
+
         // Perform zooming
         if (event->angleDelta().y() > 0) {
             if (scale_factor * zoomFactor < 500.0) {
@@ -152,6 +158,8 @@ void JFJochImage::wheelEvent(QWheelEvent *event) {
         QPointF delta = targetScenePos - updatedViewportCenter;
         translate(delta.x(), delta.y()); // Shift the view
 
+        suppress_overlay_update_ = false;
+
         updateOverlay();
         emitViewportChanged();
     }
@@ -248,8 +256,12 @@ void JFJochImage::mouseMoveEvent(QMouseEvent *event) {
             const QPoint viewDelta = event->pos() - lastMousePos;
             lastMousePos = event->pos();
 
+            // Each setValue() reaches onScroll(), so the overlay was rebuilt three times per
+            // mouse move. Suppress those and rebuild once, below.
+            suppress_overlay_update_ = true;
             horizontalScrollBar()->setValue(horizontalScrollBar()->value() - viewDelta.x());
             verticalScrollBar()->setValue(verticalScrollBar()->value() - viewDelta.y());
+            suppress_overlay_update_ = false;
 
             updateOverlay();
             emitViewportChanged();
@@ -760,6 +772,8 @@ void JFJochImage::RenderImage() {
             scanLine[x] = qRgb(c.r, c.g, c.b);
         }
     });
+
+    image_dirty_ = true;
 }
 
 void JFJochImage::centerOnSpot(QPointF point) {
@@ -779,9 +793,12 @@ void JFJochImage::applyViewport(QTransform transform, QPointF center) {
     if (m_applyingViewport || !scene())
         return;
     m_applyingViewport = true;
+    // As in wheelEvent: one rebuild, not one per scrollbar move
+    suppress_overlay_update_ = true;
     setTransform(transform);
     scale_factor = transform.m11();
     centerOn(center);
+    suppress_overlay_update_ = false;
     updateOverlay();
     m_applyingViewport = false;
 }
@@ -882,13 +899,17 @@ void JFJochImage::updateOverlay() {
     qDeleteAll(overlay_items_);
     overlay_items_.clear();
 
-    // Ensure the image item exists and is up-to-date
+    // Ensure the image item exists and is up-to-date. Refreshing it marks the whole item
+    // dirty, which forces a full repaint of the viewport, so only do it when the image
+    // really changed - not on every pan and zoom.
     if (!image_item_) {
         image_item_ = new JFJochImageItem(qimg_buffer_);
         image_item_->setZValue(0);
         scene()->addItem(image_item_);
-    } else {
+        image_dirty_ = false;
+    } else if (image_dirty_) {
         image_item_->refresh();
+        image_dirty_ = false;
     }
 
     if (scale_factor > 30.0)
diff --git a/viewer/image_viewer/JFJochImage.h b/viewer/image_viewer/JFJochImage.h
index 623f8125..7bf2f6e3 100644
--- a/viewer/image_viewer/JFJochImage.h
+++ b/viewer/image_viewer/JFJochImage.h
@@ -124,6 +124,10 @@ protected:
     // instead of queueing a full recolour per event.
     void ScheduleRenderImage();
     bool render_pending_ = false;
+    bool image_dirty_ = false;   // qimg_buffer_ changed since the item was last refreshed
+    // Set while a pan/zoom moves the scrollbars, so onScroll() does not rebuild the overlay
+    // once per scrollbar; the gesture rebuilds it once itself.
+    bool suppress_overlay_update_ = false;
     void Redraw();
     void CalcROI();