allow to move the selection panel via mouse drag

src/ui/SelectionPanel.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <QPoint>
 #include <QRect>
 #include <QWidget>
 
@@ -25,6 +26,7 @@ class BuildingIconCache;
 class ItemIconCache;
 class SelectionContent;
 class Simulation;
+class QMouseEvent;
 class QScrollArea;
 class QVBoxLayout;
 
@@ -64,6 +66,14 @@ public:
     void placeIn(const QRect& viewRect,
                  const std::vector<QRect>& occupiedRects) override;
 
+protected:
+    // The panel is moved by dragging its card header (REQ-UI-SELECTION-PANEL-DRAG). Every
+    // other mouse event over the panel is swallowed here so that none of them reaches the
+    // game world beneath (REQ-UI-SELECTION-PANEL).
+    void mousePressEvent(QMouseEvent* event) override;
+    void mouseMoveEvent(QMouseEvent* event) override;
+    void mouseReleaseEvent(QMouseEvent* event) override;
+
 private:
     void handleEvent(std::shared_ptr<const SelectionChangedEvent> event) override;
     void handleEvent(std::shared_ptr<const SelectionAnchorChangedEvent> event) override;
@@ -98,10 +108,23 @@ private:
     // scrolling view or a moving ship, the side because a card that grows must not flip
     // the panel across the object (REQ-UI-SELECTION-PANEL). The side is resolved on the
     // first placement after a new anchor, being the first point at which the panel's
-    // width is known.
+    // width is known. Dragging the panel supersedes the pair for the rest of the
+    // selection (REQ-UI-SELECTION-PANEL-DRAG).
     QRect                    m_anchorRect;
     std::optional<PanelSide> m_side;
 
+    // Where the player dragged the panel, in the game world view's coordinates, and the
+    // cursor's offset within the panel while a drag is running
+    // (REQ-UI-SELECTION-PANEL-DRAG). The desired position is kept exactly as dropped:
+    // resolving it against the view's edges and the widgets the panel steps around never
+    // writes back to it, so the panel returns to it once the room is there again. It
+    // lasts as long as the selection it was set in.
+    std::optional<QPoint> m_desiredTopLeftPx;
+    std::optional<QPoint> m_dragGrabOffsetPx;
+    // Origin of the view the panel was last placed in, which is what the two coordinate
+    // systems differ by -- the panel is a sibling of the view, not a child of it.
+    QPoint m_viewOriginPx;
+
     // Scrolls the card once it outgrows the space the panel has (REQ-UI-SELECTION-PANEL).
     // The card is a child of m_body, not of the panel itself.
     QScrollArea* m_scrollArea;