draw selection rect only if mouse has moved

src/ui/GameWorldView.cpp

@@ -148,6 +148,7 @@ GameWorldView::GameWorldView(Simulation* sim, const GameConfig* config,
     , m_debugDraw(false)
     , m_rng(std::random_device{}())
     , m_boxSelecting(false)
+    , m_boxDragMoved(false)
     , m_gameOverShown(false)
     , m_schematicChoiceShown(false)
 {
@@ -300,6 +301,10 @@ void GameWorldView::onFrame()
         {
             m_boxCurrentTile =
                 getCoordinates().widgetToTile(mapFromGlobal(QCursor::pos()));
+            // The cursor has not travelled a pixel, so the drag threshold above never
+            // trips; but the scroll has grown the box past the tile it started on,
+            // which has to become visible.
+            if (m_boxCurrentTile != m_boxStartTile) { m_boxDragMoved = true; }
         }
     }
 
@@ -420,8 +425,9 @@ void GameWorldView::paintGL()
 
 WorldRenderFrame GameWorldView::makeRenderFrame() const
 {
-    return WorldRenderFrame{m_selection, m_buildMode, m_activeBeams, m_boxSelecting,
-                            m_boxStartTile, m_boxCurrentTile, m_debugDraw};
+    return WorldRenderFrame{m_selection,    m_buildMode,      m_activeBeams,
+                            m_boxSelecting, m_boxDragMoved,   m_boxStartTile,
+                            m_boxCurrentTile, m_debugDraw};
 }
 
 // ---------------------------------------------------------------------------
@@ -1224,6 +1230,8 @@ void GameWorldView::mousePressEvent(QMouseEvent* event)
         m_boxSelecting   = true;
         m_boxStartTile   = tile;
         m_boxCurrentTile = tile;
+        m_boxStartPos    = event->pos();
+        m_boxDragMoved   = false;
         break;
 
     case ControlAction::Select:
@@ -1238,6 +1246,8 @@ void GameWorldView::mousePressEvent(QMouseEvent* event)
             m_boxSelecting    = true;
             m_boxStartTile    = tile;
             m_boxCurrentTile  = tile;
+            m_boxStartPos     = event->pos();
+            m_boxDragMoved    = false;
         }
         break;
 
@@ -1349,6 +1359,14 @@ void GameWorldView::mouseMoveEvent(QMouseEvent* event)
     const QPoint tile = coordinates.widgetToTile(event->pos());
     m_cursorWorldPos  = coordinates.widgetToWorld(event->pos());
 
+    // A press is a drag once the cursor has travelled far enough from it; below that
+    // it stays a click and shows no rectangle (REQ-UI-MULTI-SELECT).
+    if (m_boxSelecting
+        && (event->pos() - m_boxStartPos).manhattanLength() >= kBoxDragThresholdPixels)
+    {
+        m_boxDragMoved = true;
+    }
+
     if (m_buildMode.isBuilderMode())
     {
         m_buildMode.setGhostTile(tile);

src/ui/GameWorldView.h

@@ -260,6 +260,11 @@ private:
     // paused or slowed, instead of fading on wall-clock time (REQ-SHP-FIRING-BEAM).
     static constexpr Tick    kBeamLifetimeTicks      = secondsToTicks(0.3);
 
+    // How far the cursor must travel from the press position, in widget pixels
+    // (Manhattan distance), before a box drag shows its rectangle
+    // (REQ-UI-MULTI-SELECT).
+    static constexpr int     kBoxDragThresholdPixels = 2;
+
     Simulation*          m_sim;
     const GameConfig*    m_config;
     const VisualsConfig* m_visuals;
@@ -310,6 +315,15 @@ private:
     bool                  m_boxSelecting;
     QPoint                m_boxStartTile;
     QPoint                m_boxCurrentTile;
+    // Where the button went down, in widget pixels; the origin the drag threshold
+    // below measures from. Pixels, not tiles: the threshold separates a click from a
+    // drag, which is a hand-steadiness question and not a tile-sized one.
+    QPoint                m_boxStartPos;
+    // Whether the cursor has moved far enough from m_boxStartPos for this to read as
+    // a drag. Until it has, the rectangle is not drawn (REQ-UI-MULTI-SELECT): a plain
+    // click would otherwise flash a one-tile rectangle. Sticky for the rest of the
+    // drag, so coming back to the press position does not hide the rectangle again.
+    bool                  m_boxDragMoved;
 
     // Interprets this widget's key events into semantic actions and publishes them
     // (REQ-UI-HOTKEYS). Owned here for now because this is the widget that holds

src/ui/WorldRenderer.cpp

@@ -1019,8 +1019,9 @@ void WorldRenderer::drawOverlays(QPainter& painter, const WorldCoordinates& coor
         }
     }
 
-    // Box-select rectangle
-    if (frame.isBoxSelecting)
+    // Box-select rectangle. Not drawn until the drag has moved far enough to read as
+    // one, so a plain click does not flash a rectangle (REQ-UI-MULTI-SELECT).
+    if (frame.isBoxSelecting && frame.isBoxDragMoved)
     {
         const QPoint tl(std::min(frame.boxStartTile.x(), frame.boxCurrentTile.x()),
                         std::min(frame.boxStartTile.y(), frame.boxCurrentTile.y()));

src/ui/WorldRenderer.h

@@ -55,6 +55,9 @@ struct WorldRenderFrame
     const BuildModeController&     buildMode;
     const std::vector<ActiveBeam>& beams;
     bool                           isBoxSelecting;
+    // Whether that box drag has passed the movement threshold that tells it apart
+    // from a click; until it has, the rectangle is not drawn (REQ-UI-MULTI-SELECT).
+    bool                           isBoxDragMoved;
     QPoint                         boxStartTile;
     QPoint                         boxCurrentTile;
     bool                           isDebugDrawEnabled;