From 7685a84c81a9b2a33872e479740220cf1dcbee97 Mon Sep 17 00:00:00 2001
From: reuk <reuk@users.noreply.github.com>
Date: Wed, 7 Jan 2026 15:35:07 +0000
Subject: [PATCH] HWNDComponentPeer: Avoid glitches and sluggishness during
 live resize

258203706cdfc1357effe8cc752672b793cea37d introduced a problem where
live-resize of a window would cause the window content to flash and slow
down.

The cause of the errant behaviour seems to be that, during resize, many
WM_PAINT messages might be dispatched over the course of a single vblank
interval. Then, each of these paint calls was waiting for the next
vblank interval to display. This additional waiting resulted in
sluggishness, since many frames would have to be presented before the
presentation 'caught up' with the current window size. This also meant
that many consecutive frames were presented with stale window size
information.

The solution added here simply checks whether we're live resizing, and
uses the vblank instead of WM_PAINT to synchronise repaints during the
resize operation.
---
 .../native/juce_Windowing_windows.cpp         | 37 ++++++++++++++-----
 1 file changed, 28 insertions(+), 9 deletions(-)

diff --git a/modules/juce_gui_basics/native/juce_Windowing_windows.cpp b/modules/juce_gui_basics/native/juce_Windowing_windows.cpp
index fb8134f45a..366c2e77d6 100644
--- a/modules/juce_gui_basics/native/juce_Windowing_windows.cpp
+++ b/modules/juce_gui_basics/native/juce_Windowing_windows.cpp
@@ -2096,6 +2096,7 @@ public:
     }
 
     bool hasTitleBar() const                 { return (styleFlags & windowHasTitleBar) != 0; }
+    bool isSizing() const                    { return sizing; }
 
 private:
     HWND hwnd, parentToAddTo;
@@ -5077,14 +5078,22 @@ public:
 
         updateRegion.findRECTAndValidate (peer.getHWND());
 
-        for (const auto& rect : updateRegion.getRects())
-            direct2DContext->addDeferredRepaint (D2DUtilities::toRectangle (rect));
+        if (peer.isSizing())
+        {
+            for (const auto& rect : updateRegion.getRects())
+                deferredRepaints.add (D2DUtilities::toRectangle (rect));
+        }
+        else
+        {
+            for (const auto& rect : updateRegion.getRects())
+                direct2DContext->addDeferredRepaint (D2DUtilities::toRectangle (rect));
 
-       #if JUCE_DIRECT2D_METRICS
-        lastPaintStartTicks = paintStartTicks;
-       #endif
+           #if JUCE_DIRECT2D_METRICS
+            lastPaintStartTicks = paintStartTicks;
+           #endif
 
-        handleDirect2DPaint();
+            handleDirect2DPaint();
+        }
     }
 
     void repaint (const Rectangle<int>& area) override
@@ -5101,10 +5110,20 @@ public:
 
     void onVBlank() override
     {
-        for (auto deferredRect : deferredRepaints)
+        if (peer.isSizing())
         {
-            auto r = D2DUtilities::toRECT (deferredRect);
-            InvalidateRect (peer.getHWND(), &r, FALSE);
+            for (const auto& rect : deferredRepaints)
+                direct2DContext->addDeferredRepaint (rect);
+
+            handleDirect2DPaint();
+        }
+        else
+        {
+            for (auto deferredRect : deferredRepaints)
+            {
+                auto r = D2DUtilities::toRECT (deferredRect);
+                InvalidateRect (peer.getHWND(), &r, FALSE);
+            }
         }
 
         deferredRepaints.clear();