DropShadower: Fix issue with shadows disappearing in TabbedComponent

The issue was caused by DropShadower using the ComponentListener interface
to listen to its target Component's changes and creating shadow Components
only if the target was visible during the event callbacks. However it was
possible that during the events the target was not yet visible because one
of its parents was not visible. When the parent became visible it would not
trigger a callback for the observed child component.

The fix attaches a ComponentListener recursively to all parents starting
from the target and responds to each componentVisibilityChanged() event.

modules/juce_gui_basics/misc/juce_DropShadower.cpp

@@ -75,6 +75,84 @@ private:
     JUCE_DECLARE_NON_COPYABLE (ShadowWindow)
 };
 
+class ParentVisibilityChangedListener  : public ComponentListener
+{
+public:
+    ParentVisibilityChangedListener (Component& r, ComponentListener& l)
+        : root (&r), listener (&l)
+    {
+        if (auto* firstParent = root->getParentComponent())
+            updateParentHierarchy (firstParent);
+    }
+
+    ~ParentVisibilityChangedListener() override
+    {
+        for (auto& compEntry : observedComponents)
+            if (auto* comp = compEntry.get())
+                comp->removeComponentListener (this);
+    }
+
+    void componentVisibilityChanged (Component&) override
+    {
+        listener->componentVisibilityChanged (*root);
+    }
+
+    void componentParentHierarchyChanged (Component& component) override
+    {
+        if (root == &component)
+            if (auto* firstParent = root->getParentComponent())
+                updateParentHierarchy (firstParent);
+    }
+
+private:
+    class ComponentWithWeakReference
+    {
+    public:
+        explicit ComponentWithWeakReference (Component& c)
+            : ptr (&c), ref (&c) {}
+
+        Component* get() const { return ref.get(); }
+
+        bool operator< (const ComponentWithWeakReference& other) const { return ptr < other.ptr; }
+
+    private:
+        Component* ptr;
+        WeakReference<Component> ref;
+    };
+
+    void updateParentHierarchy (Component* rootComponent)
+    {
+        const auto lastSeenComponents = std::exchange (observedComponents, [&]
+        {
+            std::set<ComponentWithWeakReference> result;
+
+            for (auto node = rootComponent; node != nullptr; node = node->getParentComponent())
+                result.emplace (*node);
+
+            return result;
+        }());
+
+        const auto withDifference = [] (const auto& rangeA, const auto& rangeB, auto&& callback)
+        {
+            std::vector<ComponentWithWeakReference> result;
+            std::set_difference (rangeA.begin(), rangeA.end(), rangeB.begin(), rangeB.end(), std::back_inserter (result));
+
+            for (const auto& item : result)
+                if (auto* c = item.get())
+                    callback (*c);
+        };
+
+        withDifference (lastSeenComponents, observedComponents, [this] (auto& comp) { comp.removeComponentListener (this); });
+        withDifference (observedComponents, lastSeenComponents, [this] (auto& comp) { comp.addComponentListener (this); });
+    }
+
+    Component* root = nullptr;
+    ComponentListener* listener = nullptr;
+    std::set<ComponentWithWeakReference> observedComponents;
+
+    JUCE_DECLARE_NON_COPYABLE (ParentVisibilityChangedListener)
+    JUCE_DECLARE_NON_MOVEABLE (ParentVisibilityChangedListener)
+};
 
 //==============================================================================
 DropShadower::DropShadower (const DropShadow& ds)  : shadow (ds)  {}
@@ -109,6 +187,11 @@ void DropShadower::setOwner (Component* componentToFollow)
         updateParent();
         owner->addComponentListener (this);
 
+        // The visibility of `owner` is transitively affected by the visibility of its parents. Thus we need to trigger the
+        // componentVisibilityChanged() event in case it changes for any of the parents.
+        visibilityChangedListener = std::make_unique<ParentVisibilityChangedListener> (*owner,
+                                                                                       static_cast<ComponentListener&> (*this));
+
         updateShadows();
     }
 }

modules/juce_gui_basics/misc/juce_DropShadower.h

@@ -75,6 +75,8 @@ private:
     void updateParent();
     void updateShadows();
 
+    std::unique_ptr<ComponentListener> visibilityChangedListener;
+
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (DropShadower)
 };