Find/Replace overlay: fix Ctrl+F6 crash on Linux

Avoid being stuck in an event loop by reacting asynchronously to part
activation events.
Additionally, manually give focus to the editor part after switching.
This behavior is default on windows but not consistent on Linux.

fixes #1951

bundles/org.eclipse.ui.workbench.texteditor/src/org/eclipse/ui/internal/findandreplace/overlay/FindReplaceOverlay.java

@@ -247,21 +247,17 @@ public void shellDeactivated(ShellEvent e) {
 	private IPartListener partListener = new IPartListener() {
 		@Override
 		public void partActivated(IWorkbenchPart part) {
-			if (getShell() != null) {
-				getShell().setVisible(isPartCurrentlyDisplayedInPartSash());
-			}
+			getShell().getDisplay().asyncExec(this::adaptToPartActivationChange);
 		}
 
 		@Override
 		public void partDeactivated(IWorkbenchPart part) {
-			// Do nothing
+			getShell().getDisplay().asyncExec(this::adaptToPartActivationChange);
 		}
 
 		@Override
 		public void partBroughtToTop(IWorkbenchPart part) {
-			if (getShell() != null) {
-				getShell().setVisible(isPartCurrentlyDisplayedInPartSash());
-			}
+			getShell().getDisplay().asyncExec(this::adaptToPartActivationChange);
 		}
 
 		@Override
@@ -273,6 +269,35 @@ public void partClosed(IWorkbenchPart part) {
 		public void partOpened(IWorkbenchPart part) {
 			// Do nothing
 		}
+
+		private void adaptToPartActivationChange() {
+			if (getShell() == null || targetPart.getSite().getPart() == null) {
+				return;
+			}
+
+			boolean isOverlayVisible = isPartCurrentlyDisplayedInPartSash();
+
+			getShell().setVisible(isOverlayVisible);
+
+			if (isOverlayVisible) {
+				targetPart.getSite().getShell().setActive();
+				targetPart.setFocus();
+				getShell().getDisplay().asyncExec(this::focusTargetWidget);
+			}
+		}
+
+		private void focusTargetWidget() {
+			if (getShell() == null || targetPart.getSite().getPart() == null) {
+				return;
+			}
+			if (!(targetPart instanceof StatusTextEditor)) {
+				return;
+			}
+			StatusTextEditor textEditor = (StatusTextEditor) targetPart;
+			Control targetWidget = textEditor.getAdapter(ITextViewer.class).getTextWidget();
+
+			targetWidget.forceFocus();
+		}
 	};
 
 	private boolean isPartCurrentlyDisplayedInPartSash() {