make selection box sub-tile aware

src/ui/GameWorldView.h

@@ -226,6 +226,17 @@ private:
     // which is the only case that goes on to start a box drag.
     bool                     selectAtPoint(QPoint tile, QVector2D worldPos, bool additive);
     void                     selectInBox(bool additive);
+    // Moves the running box drag's far corner to the world position under
+    // `cursorWidgetPos` and, once the cursor sits far enough from where the anchor is
+    // drawn, promotes the gesture from a click to a drag. Every corner update goes
+    // through here, including the ones a scrolling view causes under a cursor that
+    // has not moved (REQ-UI-MULTI-SELECT).
+    void                     updateBoxDrag(QPoint cursorWidgetPos);
+    // The box the drag currently spans, in world coordinates and normalized: the
+    // rectangle between its two corners once it reads as a drag, and the whole tile
+    // the button went down on before that (REQ-UI-MULTI-SELECT). Both what is drawn
+    // and what is selected come from here, so they can never disagree.
+    QRectF                   getBoxWorldRect() const;
     // Publishes where on the screen the selection about to be made sits, so the
     // selection panel can be placed beside it (REQ-UI-SELECTION-PANEL). Called with
     // what is about to be selected, immediately before selecting it, and publishes
@@ -313,16 +324,16 @@ private:
     // Not owned; set after construction, so null until MainWindow has built it.
     const BlueprintLibrary* m_blueprintLibrary = nullptr;
     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.
+    // The drag's two corners in world coordinates, unsnapped: where the button went
+    // down and where the cursor is now (REQ-UI-MULTI-SELECT). World rather than
+    // widget coordinates so the anchor keeps the spot in the world it was placed on
+    // when the view scrolls under a held button.
+    QVector2D             m_boxStartWorld;
+    QVector2D             m_boxCurrentWorld;
+    // Whether the cursor has moved far enough from the anchor for this to read as a
+    // drag. Until it has, the rectangle is not drawn and the box resolves as the
+    // whole anchor tile (REQ-UI-MULTI-SELECT). 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