Redefine camera scroll as view center

m_scrollXTiles now stores the world-X (tiles) at the center of the
viewport instead of its left edge. A new viewLeftTiles() helper derives
the left edge for the world<->widget conversions and tile culling, so
the half-viewport math lives in one place.

The view now opens centered on the asteroid/space edge and pans left
until the buildable edge is centered and right until the enemy stations
are centered (revealing a little space beyond them).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DZR44tA8sn4dPqDzAVXyps

src/ui/GameWorldView.cpp

@@ -293,7 +293,7 @@ void GameWorldView::onFrame()
     // Apply held scroll
     {
         // Pan speed depends on where the view is centered (REQ-UI-SCROLL-SPEED).
-        const float viewCenterX = m_scrollXTiles + viewportWidthTiles() / 2.0f;
+        const float viewCenterX = m_scrollXTiles;
         const float delta = panSpeedTilesPerSecondAt(viewCenterX)
             * static_cast<float>(elapsed) / 1000.0f;
         const float scrollBefore = m_scrollXTiles;
@@ -435,10 +435,15 @@ float GameWorldView::viewportWidthTiles() const
     return static_cast<float>(width()) / tilePx();
 }
 
+float GameWorldView::viewLeftTiles() const
+{
+    return m_scrollXTiles - viewportWidthTiles() / 2.0f;
+}
+
 QPointF GameWorldView::worldToWidget(QVector2D worldPos) const
 {
     return QPointF(
-        static_cast<qreal>((worldPos.x() - m_scrollXTiles) * tilePx()),
+        static_cast<qreal>((worldPos.x() - viewLeftTiles()) * tilePx()),
         static_cast<qreal>(worldPos.y() * tilePx()));
 }
 
@@ -450,14 +455,14 @@ QPointF GameWorldView::tileToWidget(QPoint tile) const
 
 QPoint GameWorldView::widgetToTile(QPoint widgetPt) const
 {
-    const float wx = static_cast<float>(widgetPt.x()) / tilePx() + m_scrollXTiles;
+    const float wx = static_cast<float>(widgetPt.x()) / tilePx() + viewLeftTiles();
     const float wy = static_cast<float>(widgetPt.y()) / tilePx();
     return QPoint(static_cast<int>(std::floor(wx)), static_cast<int>(std::floor(wy)));
 }
 
 QVector2D GameWorldView::widgetToWorld(QPoint widgetPt) const
 {
-    const float wx = static_cast<float>(widgetPt.x()) / tilePx() + m_scrollXTiles;
+    const float wx = static_cast<float>(widgetPt.x()) / tilePx() + viewLeftTiles();
     const float wy = static_cast<float>(widgetPt.y()) / tilePx();
     return QVector2D(wx, wy);
 }
@@ -471,9 +476,9 @@ QRectF GameWorldView::tileRect(QPoint tile) const
 
 QRect GameWorldView::viewportRect() const
 {
-    const int left   = static_cast<int>(std::floor(m_scrollXTiles)) - 1;
+    const int left   = static_cast<int>(std::floor(viewLeftTiles())) - 1;
     const int top    = 0;
-    const int right  = static_cast<int>(std::ceil(m_scrollXTiles + viewportWidthTiles())) + 1;
+    const int right  = static_cast<int>(std::ceil(viewLeftTiles() + viewportWidthTiles())) + 1;
     const int bottom = m_config->world.heightTiles;
     return QRect(left, top, right - left, bottom - top);
 }
@@ -546,8 +551,11 @@ float GameWorldView::panSpeedTilesPerSecondAt(float viewCenterXTiles) const
 
 void GameWorldView::clampScroll()
 {
+    // m_scrollXTiles is the view center, so the pan limits are the edges themselves:
+    // the view can pan left until the buildable/asteroid edge is centered, and right
+    // until the enemy stations are centered (revealing a little space beyond them).
     const float leftBound  = asteroidLeftEdge();
-    const float rightBound = enemyStationRightEdge() - viewportWidthTiles();
+    const float rightBound = enemyStationRightEdge();
     m_scrollXTiles = std::max(leftBound, std::min(m_scrollXTiles, rightBound));
 }
 
@@ -906,7 +914,7 @@ void GameWorldView::drawPortGlyph(QPainter& painter, QPoint bodyTile,
 
 void GameWorldView::drawTiles(QPainter& painter)
 {
-    const int leftTile   = static_cast<int>(std::floor(m_scrollXTiles)) - 1;
+    const int leftTile   = static_cast<int>(std::floor(viewLeftTiles())) - 1;
     const int rightTile  = leftTile + static_cast<int>(std::ceil(viewportWidthTiles())) + 2;
     const int bottomTile = m_config->world.heightTiles;
 

src/ui/GameWorldView.h

@@ -134,6 +134,9 @@ private:
 
     float    tilePx() const;
     float    viewportWidthTiles() const;
+    // World-X (tiles) at the left edge of the viewport. m_scrollXTiles stores the
+    // view center; this derives the left edge the world<->widget conversions need.
+    float    viewLeftTiles() const;
     QPointF  worldToWidget(QVector2D worldPos) const;
     QPointF  tileToWidget(QPoint tile) const;
     QPoint   widgetToTile(QPoint widgetPt) const;
@@ -218,6 +221,7 @@ private:
     std::mt19937  m_rng;
     double        m_gameSpeedMultiplier;
     double        m_prevNonZeroSpeed;
+    // World-X (tiles) at the center of the viewport (see viewLeftTiles()).
     float         m_scrollXTiles;
 
     QTimer* m_renderTimer;