feat: Implement pixel-perfect camera panning

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/Camera.cpp

@@ -21,7 +21,7 @@ void Camera::mousePressEvent(QMouseEvent* event)
     m_lastPos = event->pos();
 }
 
-void Camera::mouseMoveEvent(QMouseEvent* event)
+void Camera::mouseMoveEvent(QMouseEvent* event, int viewportHeight)
 {
     int dx = event->pos().x() - m_lastPos.x();
     int dy = event->pos().y() - m_lastPos.y();
@@ -29,12 +29,17 @@ void Camera::mouseMoveEvent(QMouseEvent* event)
     if (event->buttons() & Qt::MiddleButton) {
         if (QApplication::keyboardModifiers() & Qt::ShiftModifier) {
             // Pan
+            if (viewportHeight == 0) viewportHeight = 1;
+
+            // This logic is based on a 45-degree field of view.
+            float fov = 45.0f;
             float dist = -m_zoom;
-            dist = qMin(dist, 200.0f); // Cap distance to avoid crazy fast pan.
-            // Match old speed at default zoom, and have a minimum speed.
-            float panFactor = dist * 0.000495f + 0.0001f;
-            setPanX(m_panX + dx * panFactor);
-            setPanY(m_panY - dy * panFactor);
+
+            float world_height = 2.0f * dist * tan(qDegreesToRadians(fov / 2.0f));
+            float pixels_to_world = world_height / viewportHeight;
+
+            setPanX(m_panX + dx * pixels_to_world);
+            setPanY(m_panY - dy * pixels_to_world);
         } else {
             // Rotate
             setXRotation(m_xRot + 8 * dy);

src/Camera.h

@@ -20,7 +20,7 @@ public:
     explicit Camera(QObject *parent = nullptr);
 
     void mousePressEvent(QMouseEvent* event);
-    void mouseMoveEvent(QMouseEvent* event);
+    void mouseMoveEvent(QMouseEvent* event, int viewportHeight);
     void wheelEvent(QWheelEvent* event, const QVector3D& worldPos);
 
     QMatrix4x4 modelViewMatrix() const;

src/ViewportWidget.cpp

@@ -233,7 +233,7 @@ void ViewportWidget::mouseMoveEvent(QMouseEvent *event)
         m_activeSketchTool->mouseMoveEvent(event);
     }
 
-    m_camera->mouseMoveEvent(event);
+    m_camera->mouseMoveEvent(event, height());
     update();
 }