HWNDComponentPeer: Fix reentrancy bug in DPICHANGED handler

Sometimes, changing the bounds of the window inside the DPICHANGED
handler can cause further DPI change events to be processed. Previously,
the scaleFactor set by the "inner" events was also being used when
notifying listeners about the "outer" events, leading to graphical
glitches.

An effect of the bug was that VST2 views in the AudioPluginHost would
occasionally render with an incorrect size and position after dragging
them between displays with different scale factors.

With this change in place, we only notify listeners and update window
bounds once there are no DPI changes in progress.

modules/juce_gui_basics/native/juce_win32_Windowing.cpp

@@ -1510,7 +1510,7 @@ public:
         if (! hasMoved)    flags |= SWP_NOMOVE;
         if (! hasResized)  flags |= SWP_NOSIZE;
 
-        setWindowPos (hwnd, newBounds, flags, ! isInDPIChange);
+        setWindowPos (hwnd, newBounds, flags, numInDpiChange == 0);
 
         if (hasResized && isValidPeer (this))
         {
@@ -2027,7 +2027,7 @@ private:
    #endif
 
     double scaleFactor = 1.0;
-    bool isInDPIChange = false;
+    int numInDpiChange = 0;
 
     bool isAccessibilityActive = false;
 
@@ -3371,28 +3371,33 @@ private:
 
     LRESULT handleDPIChanging (int newDPI, RECT newRect)
     {
-        auto newScale = (double) newDPI / USER_DEFAULT_SCREEN_DPI;
+        const auto newScale = (double) newDPI / USER_DEFAULT_SCREEN_DPI;
+
+        if (approximatelyEqual (scaleFactor, newScale))
+            return 0;
+
+        const auto oldScale = std::exchange (scaleFactor, newScale);
 
-        if (! approximatelyEqual (scaleFactor, newScale))
         {
-            auto oldScale = scaleFactor;
-            scaleFactor = newScale;
+            const ScopedValueSetter<int> setter (numInDpiChange, numInDpiChange + 1);
+            setBounds (windowBorder.subtractedFrom (convertPhysicalScreenRectangleToLogical (rectangleFromRECT (newRect), hwnd)), fullScreen);
+        }
 
-            {
-                const ScopedValueSetter<bool> setter (isInDPIChange, true);
-                setBounds (windowBorder.subtractedFrom (convertPhysicalScreenRectangleToLogical (rectangleFromRECT (newRect), hwnd)), fullScreen);
-            }
+        // This is to handle reentrancy. If responding to a DPI change triggers further DPI changes,
+        // we should only notify listeners and resize windows once all of the DPI changes have
+        // resolved.
+        if (numInDpiChange != 0)
+            return 0;
 
-            updateShadower();
-            InvalidateRect (hwnd, nullptr, FALSE);
+        updateShadower();
+        InvalidateRect (hwnd, nullptr, FALSE);
 
-            ChildWindowCallbackData callbackData;
-            callbackData.scaleRatio = (float) (scaleFactor / oldScale);
+        ChildWindowCallbackData callbackData;
+        callbackData.scaleRatio = (float) (scaleFactor / oldScale);
 
-            EnumChildWindows (hwnd, getChildWindowRectCallback, (LPARAM) &callbackData);
-            scaleFactorListeners.call ([&] (ScaleFactorListener& l) { l.nativeScaleFactorChanged (scaleFactor); });
-            EnumChildWindows (hwnd, scaleChildWindowCallback, (LPARAM) &callbackData);
-        }
+        EnumChildWindows (hwnd, getChildWindowRectCallback, (LPARAM) &callbackData);
+        scaleFactorListeners.call ([this] (ScaleFactorListener& l) { l.nativeScaleFactorChanged (scaleFactor); });
+        EnumChildWindows (hwnd, scaleChildWindowCallback, (LPARAM) &callbackData);
 
         return 0;
     }
@@ -4916,12 +4921,14 @@ void Desktop::allowedOrientationsChanged() {}
 static const Displays::Display* getCurrentDisplayFromScaleFactor (HWND hwnd)
 {
     Array<const Displays::Display*> candidateDisplays;
-    double scaleToLookFor = -1.0;
 
-    if (auto* peer = HWNDComponentPeer::getOwnerOfWindow (hwnd))
-        scaleToLookFor = peer->getPlatformScaleFactor();
-    else
-        scaleToLookFor = getScaleFactorForWindow (hwnd);
+    const auto scaleToLookFor = [&]
+    {
+        if (auto* peer = HWNDComponentPeer::getOwnerOfWindow (hwnd))
+            return peer->getPlatformScaleFactor();
+
+        return getScaleFactorForWindow (hwnd);
+    }();
 
     auto globalScale = Desktop::getInstance().getGlobalScaleFactor();
 
@@ -4934,12 +4941,13 @@ static const Displays::Display* getCurrentDisplayFromScaleFactor (HWND hwnd)
         if (candidateDisplays.size() == 1)
             return candidateDisplays[0];
 
-        Rectangle<int> bounds;
+        const auto bounds = [&]
+        {
+            if (auto* peer = HWNDComponentPeer::getOwnerOfWindow (hwnd))
+                return peer->getComponent().getTopLevelComponent()->getBounds();
 
-        if (auto* peer = HWNDComponentPeer::getOwnerOfWindow (hwnd))
-            bounds = peer->getComponent().getTopLevelComponent()->getBounds();
-        else
-            bounds = Desktop::getInstance().getDisplays().physicalToLogical (rectangleFromRECT (getWindowScreenRect (hwnd)));
+            return Desktop::getInstance().getDisplays().physicalToLogical (rectangleFromRECT (getWindowScreenRect (hwnd)));
+        }();
 
         const Displays::Display* retVal = nullptr;
         int maxArea = -1;