Viewer: the magnifier re-opens on the current cursor position

centerAt returns early while the window is hidden, and nothing replays the last
position when it comes back, so re-opening the magnifier showed whatever region
the cursor was over when it was closed - with current pixels, which makes it
look like a live view of the wrong place. Remember the position while hidden and
apply it on show.

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

viewer/windows/JFJochMagnifierWindow.cpp

@@ -4,6 +4,8 @@
 #include "JFJochMagnifierWindow.h"
 #include "../image_viewer/JFJochFollowerImage.h"
 
+#include <QShowEvent>
+
 JFJochMagnifierWindow::JFJochMagnifierWindow(QWidget *parent)
     : JFJochHelperWindow(parent) {
     setWindowTitle("Magnifier");
@@ -23,7 +25,16 @@ void JFJochMagnifierWindow::imageLoaded(std::shared_ptr<const JFJochReaderImage>
 }
 
 void JFJochMagnifierWindow::centerAt(QPointF scenePos) {
+    m_last_scene_pos = scenePos;
     if (!isVisible())
         return;
     m_image->CenterAt(scenePos);
 }
+
+void JFJochMagnifierWindow::showEvent(QShowEvent *event) {
+    JFJochHelperWindow::showEvent(event);
+    // setFrame keeps the pixels current while hidden, but the centre does not move, so without this
+    // the window re-opens showing wherever the cursor happened to be when it was closed.
+    if (m_last_scene_pos)
+        m_image->CenterAt(*m_last_scene_pos);
+}

viewer/windows/JFJochMagnifierWindow.h

@@ -4,6 +4,7 @@
 #pragma once
 
 #include <memory>
+#include <optional>
 
 #include "JFJochHelperWindow.h"
 #include <QPointF>
@@ -19,6 +20,10 @@ class JFJochMagnifierWindow : public JFJochHelperWindow {
     Q_OBJECT
 
     JFJochFollowerImage *m_image;
+    // Where the cursor last was. Hover positions arriving while the window is hidden are remembered
+    // rather than dropped, so re-opening shows the region the cursor is over now and not the one it
+    // was over when the window was closed.
+    std::optional<QPointF> m_last_scene_pos;
 
 public:
     explicit JFJochMagnifierWindow(QWidget *parent = nullptr);
@@ -27,6 +32,9 @@ public:
     // displayed from the frame the main view rendered, nothing is converted here.
     void imageLoaded(std::shared_ptr<const JFJochReaderImage> image) override;
 
+protected:
+    void showEvent(QShowEvent *event) override;
+
 public slots:
     void setFrame(std::shared_ptr<const QImage> frame);
     void centerAt(QPointF scenePos);