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

@@ -882,6 +882,15 @@ private void updatePlacementAndVisibility() {
 			getShell().setVisible(false);
 			return;
 		}
+		if (isInvalidTargetShell()) {
+			getShell().getDisplay().syncExec(() -> {
+				close();
+				setParentShell(targetPart.getSite().getShell());
+				open();
+				targetPart.setFocus();
+			});
+			return;
+		}
 		getShell().requestLayout();
 		if (!(targetPart instanceof StatusTextEditor textEditor)) {
 			return;
@@ -902,6 +911,17 @@ 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;