[ui] DelegateSelectionBox: Fetch children indices when box selecting backdrops

This allows selecting children explicitly during this call rather than implicitly invoking select children which was inducing a binding loop on selection property

Author: waaake

meshroom/ui/qml/Controls/DelegateSelectionBox.qml

@@ -25,6 +25,15 @@ SelectionBox {
             const delegateRect = Qt.rect(delegate.x, delegate.y, delegate.width, delegate.height);
             if (Geom2D.rectRectIntersect(mappedSelectionRect, delegateRect)) {
                 selectedIndices.push(i);
+
+                // Check if the node is a backdrop
+                // If so add all of it's children indices as well
+                if (delegate.isBackdrop) {
+                    let children = delegate.getChildrenIndices(true);
+                    for (var c = 0; c < children.length; c++) {
+                        selectedIndices.push(children[c]);
+                    }
+                }
             }
         }
         delegateSelectionEnded(selectedIndices, modifiers);