allow to move the selection panel via mouse drag

src/lib/core/FloatingPanelPlacement.cpp

@@ -2,6 +2,39 @@
 
 #include <algorithm>
 
+namespace
+{
+
+// The part every placement shares, whatever put the panel where it wants to be: it is
+// pushed inside band rather than hanging off it, shortened to what its own column has
+// free, and lifted by however much of it hangs below that. wantedLeftPx and wantedTopPx
+// are where the panel would stand if nothing were in the way.
+QRect fitInBand(const QRect& band, int wantedLeftPx, int wantedTopPx, QSize wantedSize,
+                const std::vector<QRect>& occupiedRects, int marginPx)
+{
+    const int widthPx = std::min(wantedSize.width(), band.width());
+
+    int leftPx = std::min(wantedLeftPx, band.right() - widthPx + 1);
+    leftPx     = std::max(leftPx, band.left());
+
+    // Only the widgets its own column meets can shorten it.
+    const int bottomPx = getAvailableBottomPx(band, occupiedRects, leftPx,
+                                              leftPx + widthPx - 1, marginPx);
+    const int heightPx =
+        std::min(wantedSize.height(), std::max(0, bottomPx - band.top() + 1));
+
+    // Never above the band: a panel taller than the space left is capped instead, and
+    // scrolls.
+    int topPx = std::max(wantedTopPx, band.top());
+    topPx     = std::min(topPx, bottomPx - heightPx + 1);
+    topPx     = std::max(topPx, band.top());
+
+    return QRect(leftPx, topPx, widthPx, heightPx);
+}
+
+}  // namespace
+
+
 int getAvailableBottomPx(const QRect& band, const std::vector<QRect>& occupiedRects,
                          int leftPx, int rightPx, int marginPx)
 {
@@ -51,26 +84,27 @@ QRect placeBesideAnchor(const QRect& band, const QRect& anchorRect, PanelSide si
     const int widthPx = std::min(wantedSize.width(), band.width());
 
     // Against the anchor on the chosen side, growing away from it: the edge facing the
-    // selection is the one that stays put as the panel's content resizes.
-    int leftPx = (side == PanelSide::Right) ? anchorRect.right() + marginPx + 1
-                                            : anchorRect.left() - marginPx - widthPx;
-    // A panel that does not fit there is pushed back inside the view rather than hanging
-    // off it, which is what puts it over the selection when neither side had room.
-    leftPx = std::min(leftPx, band.right() - widthPx + 1);
-    leftPx = std::max(leftPx, band.left());
-
-    // Only the widgets its own column meets can shorten it.
-    const int bottomPx = getAvailableBottomPx(band, occupiedRects, leftPx,
-                                              leftPx + widthPx - 1, marginPx);
-    const int heightPx =
-        std::min(wantedSize.height(), std::max(0, bottomPx - band.top() + 1));
+    // selection is the one that stays put as the panel's content resizes. A panel that
+    // does not fit there is pushed back inside the view rather than hanging off it, which
+    // is what puts it over the selection when neither side had room.
+    const int wantedLeftPx = (side == PanelSide::Right)
+                                 ? anchorRect.right() + marginPx + 1
+                                 : anchorRect.left() - marginPx - widthPx;
 
     // Top-aligned with the anchor, then lifted by however much of it hangs below what is
-    // free. Never above the band: a panel taller than the space left is capped instead,
-    // and scrolls.
-    int topPx = std::max(anchorRect.top(), band.top());
-    topPx     = std::min(topPx, bottomPx - heightPx + 1);
-    topPx     = std::max(topPx, band.top());
-
-    return QRect(leftPx, topPx, widthPx, heightPx);
+    // free.
+    return fitInBand(band, wantedLeftPx, anchorRect.top(), wantedSize, occupiedRects,
+                     marginPx);
+}
+
+QRect placeAtDesiredTopLeft(const QRect& band, const QPoint& desiredTopLeftPx,
+                            QSize wantedSize, const std::vector<QRect>& occupiedRects,
+                            int marginPx)
+{
+    // Where the player dropped it, resolved by the same rules as any other placement
+    // (REQ-UI-SELECTION-PANEL-DRAG). Nothing here is written back to the desired point:
+    // a panel lifted above the build button bar returns to where it was dropped as soon
+    // as the bar stops meeting its column.
+    return fitInBand(band, desiredTopLeftPx.x(), desiredTopLeftPx.y(), wantedSize,
+                     occupiedRects, marginPx);
 }

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);