draw selection rect only if mouse has moved

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