Viewer: lay out pixel labels only for the area being repainted

drawPixelLabels took its range from the whole viewport rather than from the
exposed rect it was given, so a 200x40 px hover repaint still walked up to 5000
cells doing mapFromScene + QImage::pixel + drawText for each, only to have the
result clipped away. With hover feedback now rate-limited to 15 Hz that ran ~75
times a second at high zoom, against once per overlay rebuild before the
rendering rework. Intersect with the exposed rect; same in the magnifier.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

viewer/image_viewer/JFJochFollowerImage.cpp

@@ -91,7 +91,8 @@ void JFJochFollowerImage::drawForeground(QPainter *painter, const QRectF &rect)
     if (static_cast<int64_t>(pixels.size()) < static_cast<int64_t>(W) * H)
         return;   // values belong to a different frame
 
-    const QRectF visible = mapToScene(viewport()->rect()).boundingRect();
+    // Clipped to the exposed area, for the same reason as JFJochImage::drawPixelLabels.
+    const QRectF visible = mapToScene(viewport()->rect()).boundingRect() & rect;
     const int x0 = std::max(0, static_cast<int>(std::floor(visible.left())));
     const int x1 = std::min(W, static_cast<int>(std::ceil(visible.right())));
     const int y0 = std::max(0, static_cast<int>(std::floor(visible.top())));

viewer/image_viewer/JFJochImage.cpp

@@ -852,10 +852,13 @@ QString JFJochImage::PixelLabel(int x, int y) const {
     return QString::number(val, 'e', 1);
 }
 
-void JFJochImage::drawPixelLabels(QPainter *painter) {
+void JFJochImage::drawPixelLabels(QPainter *painter, const QRectF &rect) {
     constexpr int kMaxLabels = 5000;
 
-    const QRectF visibleRect = mapToScene(viewport()->rect()).boundingRect();
+    // Only the exposed area, not the whole viewport: a hover repaint dirties a couple of hundred
+    // pixels, and laying out every visible cell for it would do thousands of mapFromScene / pixel /
+    // drawText calls whose output is then clipped away - at up to 15 repaints a second.
+    const QRectF visibleRect = mapToScene(viewport()->rect()).boundingRect() & rect;
     const int startX = std::max(0, static_cast<int>(std::floor(visibleRect.left())));
     const int endX   = std::min(static_cast<int>(W), static_cast<int>(std::ceil(visibleRect.right())));
     const int startY = std::max(0, static_cast<int>(std::floor(visibleRect.top())));
@@ -895,7 +898,7 @@ void JFJochImage::drawForeground(QPainter *painter, const QRectF &rect) {
     QGraphicsView::drawForeground(painter, rect);
 
     if (scale_factor > 30.0 && W * H > 0 && frame_ && !frame_->isNull())
-        drawPixelLabels(painter);
+        drawPixelLabels(painter, rect);
 }
 
 void JFJochImage::resetScenePointers() {

viewer/image_viewer/JFJochImage.h

@@ -82,7 +82,7 @@ class JFJochImage : public QGraphicsView {
     void DrawROI();
     virtual void addCustomOverlay();
     void updateROI();
-    void drawPixelLabels(QPainter *painter);
+    void drawPixelLabels(QPainter *painter, const QRectF &rect);
     void wheelEvent(QWheelEvent* event) override;
     void resizeEvent(QResizeEvent *event) override;
     void contextMenuEvent(QContextMenuEvent *event) override;