fix issue where scrolling while drawing selection box did not update selection box size

src/ui/GameWorldView.cpp

@@ -9,6 +9,7 @@
 #include <string>
 
 #include <QColor>
+#include <QCursor>
 #include <QFont>
 #include <QKeyEvent>
 #include <QMessageBox>
@@ -183,9 +184,17 @@ void GameWorldView::onFrame()
     {
         const float delta = kScrollSpeedTilesPerSec
             * static_cast<float>(elapsed) / 1000.0f;
+        const float scrollBefore = m_scrollXTiles;
         if (m_scrollLeft)  { m_scrollXTiles -= delta; }
         if (m_scrollRight) { m_scrollXTiles += delta; }
         clampScroll();
+
+        // While the view scrolls, the tile under a stationary cursor changes,
+        // so refresh the box-select rectangle even though no mouse move fires.
+        if (m_boxSelecting && m_scrollXTiles != scrollBefore)
+        {
+            m_boxCurrentTile = widgetToTile(mapFromGlobal(QCursor::pos()));
+        }
     }
 
     // Fire events for any state that changed since the last frame