Find/Replace overlay: handle the case when moving editor between windows

When moving the targeted editor between different windows, the overlay
is closed to allow a retargetting.

fixes #1945

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

@@ -151,7 +151,6 @@ public FindReplaceOverlay(Shell parent, IWorkbenchPart part, IFindReplaceTarget
 		setShellStyle(SWT.MODELESS);
 		setBlockOnOpen(false);
 		targetPart = part;
-
 	}
 
 	@Override
@@ -840,9 +839,17 @@ private void repositionTextSelection() {
 	}
 
 	private void updatePlacementAndVisibility() {
+		if (isInvalidTargetShell()) {
+			getShell().getDisplay().syncExec(() -> {
+				close();
+				setParentShell(targetPart.getSite().getShell());
+				open();
+				targetPart.setFocus();
+			});
+			return;
+		}
 		if (!isTargetVisible) {
 			getShell().setVisible(false);
-			return;
 		}
 		getShell().requestLayout();
 		if (!(targetPart instanceof StatusTextEditor textEditor)) {
@@ -864,6 +871,17 @@ private void updatePlacementAndVisibility() {
 		repositionTextSelection();
 	}
 
+	private boolean isInvalidTargetPart() {
+		return targetPart == null || targetPart.getSite() == null || targetPart.getSite().getShell() == null;
+	}
+
+	private boolean isInvalidTargetShell() {
+		if (isInvalidTargetPart()) {
+			return false;
+		}
+		return getShell() == null || !targetPart.getSite().getShell().equals(getShell().getParent());
+	}
+
 	private Rectangle calculateAbsoluteControlBounds(Control control) {
 		Rectangle localControlBounds = control.getBounds();
 		int width = localControlBounds.width;