Viewer: Alt and the wheel step through the images

The wheel already zooms, and with Ctrl or Shift it moves the foreground; Alt
now moves through the dataset, one image per notch, wheel up forward - the
direction QAbstractSlider's own wheel handling uses, which is what the
toolbar's scrub slider follows.

The view does not know which image it is showing, so it emits the step and the
navigation toolbar applies it: the same loadImage() every other control ends
in, with the same clamp and the same Sum setting, so nothing about loading is
duplicated. An empty dataset is left alone, because loadImage(-1) does not mean
"before the first image" but "the latest one" when the viewer is following a
running collection over HTTP.

The signal is on JFJochImage, so the other views emit it too; only the
diffraction view is connected, which leaves them as they were.

Note for a desktop where Alt+wheel does nothing: many window managers grab
Alt-modified mouse events before the application sees them, and that is a
window-manager setting, not something the viewer can take back.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FBumeJVx4oeXxiBRpkrE5H

docs/CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 ## 1.0.0
+### 1.0.0-rc.166
+* In `jfjoch_viewer`, Alt and the mouse wheel step through the dataset one image at a time.
+
 ### 1.0.0-rc.165
 This is an UNSTABLE release. It includes many experimental features, as well as many AI generated fixes. We recommend using rc.152 for production use.
 

viewer/JFJochViewerWindow.cpp

@@ -224,6 +224,8 @@ JFJochViewerWindow::JFJochViewerWindow(QWidget *parent, bool dbus, const QString
 
     connect(toolBarImage, &JFJochViewerToolbarImage::loadImage, reading_worker, &JFJochImageReadingWorker::LoadImage);
 
+    connect(viewer, &JFJochDiffractionImage::stepImage, toolBarImage, &JFJochViewerToolbarImage::stepImage);
+
     connect(toolBarDisplay, &JFJochViewerToolbarDisplay::setForeground, viewer,
             &JFJochDiffractionImage::changeForeground);
 

viewer/image_viewer/JFJochImage.cpp

@@ -143,6 +143,16 @@ void JFJochImage::setFeatureColor(QColor input) {
 void JFJochImage::wheelEvent(QWheelEvent *event) {
     if (!scene()) return;
 
+    // Alt+wheel steps through the dataset instead of zooming. The view does not know which image
+    // it shows, so the step is emitted and the navigation toolbar applies it.
+    if (event->modifiers() & Qt::AltModifier) {
+        const int delta = event->angleDelta().y();
+        if (delta != 0)
+            emit stepImage(delta > 0 ? 1 : -1);
+        event->accept();
+        return;
+    }
+
     const double zoomFactor = 1.15; // Zoom factor
 
     // Get the position of the mouse in scene coordinates

viewer/image_viewer/JFJochImage.h

@@ -230,6 +230,8 @@ signals:
     void hoverScenePos(QPointF scenePos);
     // A new frame has been rendered into Frame(). Follower views repaint on this.
     void frameRendered();
+    // Alt+wheel asks for a move through the dataset: +1 one image forward, -1 one back.
+    void stepImage(int steps);
 private slots:
     void onScroll(int value);
 public slots:

viewer/toolbar/JFJochViewerToolbarImage.cpp

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include "JFJochViewerToolbarImage.h"
+#include <algorithm>
 #include <QTimer>
 #include "../widgets/ToolbarIcons.h"
 
@@ -176,6 +177,15 @@ void JFJochViewerToolbarImage::leftmostButtonPressed() {
     emit loadImage(0, sum);
 }
 
+void JFJochViewerToolbarImage::stepImage(int steps) {
+    if (image_count_in_dataset == 0)
+        return;
+    const auto target = std::clamp<int64_t>(curr_image + steps, 0,
+                                            static_cast<int64_t>(image_count_in_dataset) - 1);
+    if (target != curr_image)
+        emit loadImage(target, sum);
+}
+
 void JFJochViewerToolbarImage::imageNumberSliderPressed() {
     image_number_slider_manual = true;
 }

viewer/toolbar/JFJochViewerToolbarImage.h

@@ -57,6 +57,7 @@ public:
     explicit JFJochViewerToolbarImage(QWidget *parent = nullptr);
 public slots:
     void setImageNumber(int64_t total_images, int64_t current_image);
+    void stepImage(int steps);
     void setAutoloadMode(JFJochImageReadingWorker::AutoloadMode input);
     void setHttpConnection(bool connected, QString addr);
 private slots: