To handle cases where text is already selected in the editor at the time of first effect firing

packages/editor/src/client/services/unified-merge-view/index.ts

@@ -4,6 +4,7 @@ import {
   acceptChunk,
   rejectChunk,
 } from '@codemirror/merge';
+import type { ViewUpdate } from '@codemirror/view';
 import { EditorView } from '@codemirror/view';
 
 import type { UseCodeMirrorEditor } from '..';
@@ -19,18 +20,27 @@ export const rejectChange = (view: EditorView): boolean => {
 
 type OnSelected = (selectedText: string, selectedTextFirstLineNumber: number) => void
 
+const processSelectedText = (editorView: EditorView | ViewUpdate, onSelected?: OnSelected) => {
+  const selection = editorView.state.selection.main;
+  const selectedText = editorView.state.sliceDoc(selection.from, selection.to);
+  const selectedTextFirstLineNumber = editorView.state.doc.lineAt(selection.from).number - 1; // 0-based line number;
+  onSelected?.(selectedText, selectedTextFirstLineNumber);
+};
+
 export const useTextSelectionEffect = (codeMirrorEditor?: UseCodeMirrorEditor, onSelected?: OnSelected): void => {
   useEffect(() => {
     if (codeMirrorEditor == null) {
       return;
     }
 
+    // To handle cases where text is already selected in the editor at the time of first effect firing
+    if (codeMirrorEditor.view != null) {
+      processSelectedText(codeMirrorEditor.view, onSelected);
+    }
+
     const extension = EditorView.updateListener.of((update) => {
       if (update.selectionSet) {
-        const selection = update.state.selection.main;
-        const selectedText = update.state.sliceDoc(selection.from, selection.to);
-        const selectedTextFirstLineNumber = update.state.doc.lineAt(selection.from).number;
-        onSelected?.(selectedText, selectedTextFirstLineNumber);
+        processSelectedText(update, onSelected);
       }
     });