From edde940e393ab7f623cc6baeaeb87f71d62f34b9 Mon Sep 17 00:00:00 2001
From: "Klare, Heiko" <Heiko.Klare@vector.com>
Date: Fri, 9 Jan 2026 18:37:34 +0100
Subject: [PATCH] [Win32] Process DPI change events via OS instead of
 Display#asyncExec

Asynchronous processing of DPI change events is currently done by
processing the event for each control via Display#asyncExec. This has
several drawbacks:
- the events are processed rather late (even after earlier scheduled,
lower-priority business events
- the events may not be processed while dragging a shell to another
monitor in case a browser currently has focus
- the events are not processed at all if a Synchronizer is used that
does not process async events (such as for an Eclipse workspace during
startup)

This change replaces the usage of Display#asyncExec for asynchronous
processing of DPI events with the a timer message beings sent with a
callback to the processed operation via the OS. WM_TIMER
events are used as they are processed with low priority by the OS but
still with higher priority by SWT than any business-logic-side tasks
scheduled at a Display instance.
---
 .../org/eclipse/swt/widgets/Control.java      | 46 ++++++++++++++++++-
 1 file changed, 45 insertions(+), 1 deletion(-)

diff --git a/bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Control.java b/bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Control.java
index 4b8c7ad85d..33e02b03f2 100644
--- a/bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Control.java	
+++ b/bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Control.java	
@@ -15,6 +15,7 @@
 package org.eclipse.swt.widgets;
 
 
+import java.lang.reflect.*;
 import java.util.*;
 import java.util.concurrent.atomic.*;
 import java.util.stream.*;
@@ -5992,7 +5993,7 @@ private void process(Control control, Runnable operation) {
 			asyncExec &= (comp.layout != null);
 		}
 		if (asyncExec) {
-			control.getDisplay().asyncExec(operation::run);
+			DPIChangeProcessingCallback.schedule(control, operation);
 		} else {
 			operation.run();
 		}
@@ -6010,6 +6011,49 @@ private boolean decrement() {
 	}
 }
 
+private static class DPIChangeProcessingCallback  {
+	private final Runnable operation;
+	private final long address;
+
+	private DPIChangeProcessingCallback(Control control, Runnable dpiChangeProcessing) {
+		// Has to have TIMERPROC signature, see https://learn.microsoft.com/en-us/windows/win32/api/winuser/nc-winuser-timerproc
+		Callback callback = new Callback(this, "run", void.class, new Type[] { int.class, int.class, int.class, int.class} );
+		this.operation = () -> {
+			if (!control.isDisposed()) {
+				dpiChangeProcessing.run();
+			}
+			callback.dispose();
+		};
+		this.address = callback.getAddress();
+	}
+
+	@SuppressWarnings("unused") // Executed as callback method referenced by signature description
+	public void run(int hwnd, int msg , int idEvent, int dwTime) {
+		OS.KillTimer(hwnd, idEvent);
+		operation.run();
+	}
+
+	private long getAddress() {
+		return address;
+	}
+
+	static void schedule(Control control, Runnable runnable) {
+		try {
+			DPIChangeProcessingCallback callback = new DPIChangeProcessingCallback(control, runnable);
+			OS.SetTimer(0, 0, 0, callback.getAddress());
+		} catch  (SWTError error) {
+			// In case of too many callbacks, fall back to running the operation synchronously
+			if (error.code == SWT.ERROR_NO_MORE_CALLBACKS) {
+				runnable.run();
+			} else {
+				throw error;
+			}
+		}
+
+	}
+
+}
+
 void sendZoomChangedEvent(Event event, Shell shell) {
 	if (event.data instanceof DPIChangeExecution dpiExecData) {
 		dpiExecData.increment();