viewer: don't rebuild the overlay while panning

Overlay items live in scene coordinates, so QGraphicsView translates them
natively as the view scrolls - no rebuild is required to keep them in the
right place during a pan. Rebuilding on every scroll signal only re-ran the
spot/prediction viewport culling, at the cost of tearing down and recreating
every overlay item on each mouse-move step. Over remote X forwarding that
per-step rebuild (and the rasterised pixels it dirties) is the dominant
source of pan lag.

Skip the rebuild while a pan drag is in progress: onScroll() returns early
during a Panning gesture and the pan branch of mouseMoveEvent no longer
schedules one. The overlay is refreshed once when the pan ends, via the
existing mouseReleaseEvent -> updateROI() -> updateOverlay() path, which
re-runs culling for the revealed viewport. Trade-off: spots/predictions in
newly revealed regions appear on release rather than mid-drag.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

viewer/image_viewer/JFJochImage.cpp

@@ -43,6 +43,15 @@ JFJochImage::JFJochImage(QWidget *parent) : QGraphicsView(parent) {
 }
 
 void JFJochImage::onScroll(int value) {
+    // While the user drags a pan, the scene scrolls natively and overlay items
+    // (which live in scene coordinates) move with it - no rebuild is needed. A
+    // single pan step emits up to two scrollbar valueChanged signals; rebuilding
+    // the overlay on each would tear down and recreate every item for no visual
+    // change - the dominant lag over remote X. The overlay is refreshed once
+    // when the pan ends (mouseReleaseEvent -> updateROI -> updateOverlay), which
+    // re-runs spot/prediction culling for the newly revealed viewport.
+    if (mouse_event_type == MouseEventType::Panning)
+        return;
     scheduleOverlayUpdate();
 }
 
@@ -221,7 +230,9 @@ void JFJochImage::mouseMoveEvent(QMouseEvent *event) {
             horizontalScrollBar()->setValue(horizontalScrollBar()->value() - viewDelta.x());
             verticalScrollBar()->setValue(verticalScrollBar()->value() - viewDelta.y());
 
-            scheduleOverlayUpdate();
+            // No overlay rebuild while dragging: the scene scrolls natively and
+            // items move with it. onScroll() skips rebuilds during a pan and the
+            // overlay is refreshed once on mouse release.
             emitViewportChanged();
             break;
         }