diff --git a/org.eclipse.wb.core.java/src/org/eclipse/wb/internal/core/editor/UndoManager.java b/org.eclipse.wb.core.java/src/org/eclipse/wb/internal/core/editor/UndoManager.java
index 7b1a5b120f..77ada4b763 100644
--- a/org.eclipse.wb.core.java/src/org/eclipse/wb/internal/core/editor/UndoManager.java
+++ b/org.eclipse.wb.core.java/src/org/eclipse/wb/internal/core/editor/UndoManager.java
@@ -84,13 +84,28 @@ public UndoManager(DesignPage designPage, ICompilationUnit unit) throws Exceptio
 	public void activate() {
 		if (!m_active) {
 			m_active = true;
-			if (!Objects.equals(m_currentSource, m_buffer.getContents())) {
-				refreshDesignerEditor();
-			}
+			refreshIfSourceChanged();
 			addBufferListener();
 		}
 	}
 
+	/**
+	 * Reparses if the buffer no longer matches the parsed source, waiting for a running parse first.
+	 */
+	private void refreshIfSourceChanged() {
+		if (!m_active) {
+			return;
+		}
+		// the progress dialog of a running parse spins the event loop, so wait for it to finish
+		if (m_designPage.getDesignerState() == DesignerState.Parsing) {
+			Display.getDefault().timerExec(1, this::refreshIfSourceChanged);
+			return;
+		}
+		if (!Objects.equals(m_currentSource, m_buffer.getContents())) {
+			refreshDesignerEditor();
+		}
+	}
+
 	/**
 	 * Deactivates {@link UndoManager}, so it stops listening for buffer changes.
 	 */
diff --git a/org.eclipse.wb.core/src/org/eclipse/wb/internal/core/utils/execution/ExecutionUtils.java b/org.eclipse.wb.core/src/org/eclipse/wb/internal/core/utils/execution/ExecutionUtils.java
index 7398f2260e..20517f8fac 100644
--- a/org.eclipse.wb.core/src/org/eclipse/wb/internal/core/utils/execution/ExecutionUtils.java
+++ b/org.eclipse.wb.core/src/org/eclipse/wb/internal/core/utils/execution/ExecutionUtils.java
@@ -184,15 +184,22 @@ public void run() {
 	}
 
 	/**
-	 * Runs given {@link RunnableEx} inside of UI thread, using {@link Display#syncExec(Runnable)}.
+	 * Runs given {@link RunnableEx} inside of UI thread, using {@link Display#syncExec(Runnable)},
+	 * and re-throws its exceptions in the calling thread.
 	 */
 	public static void runRethrowUI(final RunnableEx runnable) {
-		Display.getDefault().syncExec(new Runnable() {
-			@Override
-			public void run() {
-				ExecutionUtils.runRethrow(runnable);
+		// the workbench's display exception handler swallows what a syncExec runnable throws
+		Throwable[] failure = new Throwable[1];
+		Display.getDefault().syncExec(() -> {
+			try {
+				runnable.run();
+			} catch (Throwable e) {
+				failure[0] = e;
 			}
 		});
+		if (failure[0] != null) {
+			throw ReflectionUtils.propagate(failure[0]);
+		}
 	}
 
 	/**
diff --git a/org.eclipse.wb.tests/src/org/eclipse/wb/tests/designer/core/util/execution/ExecutionUtilsTest.java b/org.eclipse.wb.tests/src/org/eclipse/wb/tests/designer/core/util/execution/ExecutionUtilsTest.java
index 8ee72cd24b..f05f9e9191 100644
--- a/org.eclipse.wb.tests/src/org/eclipse/wb/tests/designer/core/util/execution/ExecutionUtilsTest.java
+++ b/org.eclipse.wb.tests/src/org/eclipse/wb/tests/designer/core/util/execution/ExecutionUtilsTest.java
@@ -31,6 +31,8 @@
 import java.beans.Beans;
 import java.util.concurrent.Callable;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Consumer;
 
 /**
  * Tests for {@link ExecutionUtils} and {@link RunnableEx}.
@@ -458,6 +460,38 @@ public void test_runRethrowUI() throws Exception {
 		}
 	}
 
+	/**
+	 * Test for {@link ExecutionUtils#runRethrowUI(RunnableEx)} when the display's exception handler
+	 * swallows exceptions, as the workbench's does.
+	 */
+	@Test
+	public void test_runRethrowUI_exception() throws Exception {
+		Display display = Display.getCurrent();
+		Consumer<RuntimeException> oldHandler = display.getRuntimeExceptionHandler();
+		display.setRuntimeExceptionHandler(e -> {
+		});
+		try {
+			IllegalStateException exception = new IllegalStateException();
+			AtomicReference<Throwable> caught = new AtomicReference<>();
+			Thread thread = new Thread(() -> {
+				try {
+					ExecutionUtils.runRethrowUI(() -> {
+						throw exception;
+					});
+				} catch (Throwable e) {
+					caught.set(e);
+				}
+			});
+			thread.start();
+			while (thread.isAlive()) {
+				waitEventLoop(0);
+			}
+			assertSame(exception, caught.get());
+		} finally {
+			display.setRuntimeExceptionHandler(oldHandler);
+		}
+	}
+
 	/**
 	 * Test for {@link ExecutionUtils#runAsync(RunnableEx)}.
 	 */