Masks as Photoshop's: the button reveals the selection, Alt-click shows a mask alone (13.12)

src/UI/NativeLayerList.h

@@ -43,8 +43,8 @@ public:
     LayerThumbnailButton(NativeLayerList &list, bool maskTarget, QWidget *parent);
     QUuid layerID;
     const bool isMaskTarget;
-    // An accent ring while the target is selected.
-    void setTargeted(bool targeted);
+    // An accent ring while targeted; white while shown alone.
+    void setTargeted(bool targeted, bool alone = false);
     bool isTargeted() const { return m_targeted; }
 
     ~LayerThumbnailButton() override;
@@ -63,6 +63,7 @@ private:
 
     NativeLayerList &m_list;
     bool m_targeted = false;
+    bool m_alone = false;
     // Alt on a mask: chosen on release, or dragged.
     std::optional<QPoint> m_altMaskPress;
 };