Since only 1 message can be edited instantiate just 1 instance of ChatDocumentHandler and update when a message is edited.

src/qml/Component/Timeline/MessageDelegate.qml

@@ -28,6 +28,14 @@ TimelineContainer {
             Layout.fillWidth: true
             messageId: model.eventId
             visible: currentRoom.chatBoxEditId === model.eventId
+            onVisibleChanged: {
+                if (visible) {
+                    editChatDocumentHandler.document = textDocument
+                    editChatDocumentHandler.cursorPosition = cursorPosition
+                    editChatDocumentHandler.selectionStart = selectionStart
+                    editChatDocumentHandler.selectionEnd = selectionEnd
+                }
+            }
         }
         LinkPreviewDelegate {
             Layout.fillWidth: true

src/qml/Component/Timeline/MessageEditComponent.qml

@@ -124,16 +124,6 @@ QQC2.TextArea {
         }
     }
 
-    ChatDocumentHandler {
-        id: documentHandler
-        isEdit: true
-        document: root.textDocument
-        cursorPosition: root.cursorPosition
-        selectionStart: root.selectionStart
-        selectionEnd: root.selectionEnd
-        room: currentRoom // We don't care about saving for edits so this is OK.
-    }
-
     TextMetrics {
         id: editTextMetrics
         text: root.text

src/qml/Page/RoomPage.qml

@@ -26,6 +26,11 @@ Kirigami.ScrollablePage {
     /// cancel implementation.
     property bool disableCancelShortcut: false
 
+    property var editChatDocumentHandler: ChatDocumentHandler {
+        isEdit: true
+        room: currentRoom // We don't care about saving for edits so this is OK.
+    }
+
     title: currentRoom.displayName
 
     KeyNavigation.left: pageStack.get(0)