allow to move the selection panel via mouse drag

src/lib/core/FloatingPanelPlacement.h

@@ -2,6 +2,7 @@
 
 #include <vector>
 
+#include <QPoint>
 #include <QRect>
 #include <QSize>
 
@@ -41,3 +42,13 @@ PanelSide chooseSide(const QRect& band, const QRect& anchorRect, int widthPx,
 QRect placeBesideAnchor(const QRect& band, const QRect& anchorRect, PanelSide side,
                         QSize wantedSize, const std::vector<QRect>& occupiedRects,
                         int marginPx);
+
+// Where a panel of wantedSize stands once the player has dragged it to desiredTopLeftPx:
+// at that point, by the same rules that place it beside a selection -- pushed inside band,
+// lifted above whatever occupies its column, and capped in height where that leaves too
+// little room (REQ-UI-SELECTION-PANEL-DRAG). The desired point is an input only: it is
+// never corrected and handed back, which is what lets the caller keep it exactly as
+// dropped and return to it once the room is there again.
+QRect placeAtDesiredTopLeft(const QRect& band, const QPoint& desiredTopLeftPx,
+                            QSize wantedSize, const std::vector<QRect>& occupiedRects,
+                            int marginPx);