Windows: Fix issue where windows could be given incorrect size for their current scale

This issue affected windows with JUCE titlebars. When dragging the
window back and forth between two displays with different DPI settings,
the window could sometimes end up with the wrong size.

It looks like setting new window bounds inside the WM_WINDOWPOSCHANGED
message was occasionally causing the system to lose track of the correct
scale for the window.

As a byproduct of this change, JUCE should no longer receive reentrant
calls to handleDPIChanging.

modules/juce_gui_basics/native/juce_win32_Windowing.cpp

@@ -1583,6 +1583,13 @@ public:
 
     void setBounds (const Rectangle<int>& bounds, bool isNowFullScreen) override
     {
+        // If we try to set new bounds while handling an existing position change,
+        // Windows may get confused about our current scale and size.
+        // This can happen when moving a window between displays, because the mouse-move
+        // generator in handlePositionChanged can cause the window to move again.
+        if (inHandlePositionChanged)
+            return;
+
         const ScopedValueSetter<bool> scope (shouldIgnoreModalDismiss, true);
 
         fullScreen = isNowFullScreen;
@@ -1608,7 +1615,7 @@ public:
         if (! hasMoved)    flags |= SWP_NOMOVE;
         if (! hasResized)  flags |= SWP_NOSIZE;
 
-        setWindowPos (hwnd, newBounds, flags, numInDpiChange == 0);
+        setWindowPos (hwnd, newBounds, flags, ! inDpiChange);
 
         if (hasResized && isValidPeer (this))
         {
@@ -2125,7 +2132,7 @@ private:
    #endif
 
     double scaleFactor = 1.0;
-    int numInDpiChange = 0;
+    bool inDpiChange = 0, inHandlePositionChanged = 0;
 
     bool isAccessibilityActive = false;
 
@@ -3448,6 +3455,8 @@ private:
 
         if (contains (pos.roundToInt(), false))
         {
+            const ScopedValueSetter<bool> scope (inHandlePositionChanged, true);
+
             if (! areOtherTouchSourcesActive())
                 doMouseEvent (pos, MouseInputSource::invalidPressure);
 
@@ -3477,15 +3486,27 @@ private:
         scaleFactor = newScale;
 
         {
-            const ScopedValueSetter<int> setter (numInDpiChange, numInDpiChange + 1);
-            setBounds (windowBorder.subtractedFrom (convertPhysicalScreenRectangleToLogical (rectangleFromRECT (newRect), hwnd)), fullScreen);
+            const ScopedValueSetter<bool> setter (inDpiChange, true);
+            SetWindowPos (hwnd,
+                          nullptr,
+                          newRect.left,
+                          newRect.top,
+                          newRect.right  - newRect.left,
+                          newRect.bottom - newRect.top,
+                          SWP_NOZORDER | SWP_NOACTIVATE);
         }
 
         // 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)
+        if (inDpiChange)
+        {
+            // Danger! Re-entrant call to handleDPIChanging.
+            // Please report this issue on the JUCE forum, along with instructions
+            // so that a JUCE developer can reproduce the issue.
+            jassertfalse;
             return 0;
+        }
 
         updateShadower();
         InvalidateRect (hwnd, nullptr, FALSE);