fix(json-patch-ot): merge equal str_ins operations at same position

Author: atsjo

packages/json-joy/src/json-patch-ot/__tests__/scenarios.spec.ts

@@ -785,6 +785,14 @@ const groups: ScenarioGroup[] = [
         user2: [{op: 'str_ins', path: '/a', pos: 2, str: '_bar_'}],
         docEnd: {a: '12_foo__bar_345'},
       },
+      {
+        // changed: discard multiple same inserts (will typically happen in markdown task lists)
+        name: 'Merge equal inserts at same position.',
+        docStart: {a: '12345'},
+        user1: [{op: 'str_ins', path: '/a', pos: 2, str: '_foo_'}],
+        user2: [{op: 'str_ins', path: '/a', pos: 2, str: '_foo_'}],
+        docEnd: {a: '12_foo_345'},
+      },
     ],
   },
   {

packages/json-joy/src/json-patch-ot/transforms/xStrIns.ts

@@ -6,6 +6,7 @@ export const xStrIns = (ins: OpStrIns, op: Op): null | Op | Op[] => {
   if (op instanceof OpStrIns) {
     if (!isPathEqual(ins.path, op.path)) return op;
     if (ins.pos > op.pos) return op;
+    if (ins.pos === op.pos && ins.str === op.str) return null;
     return operationToOp({...op.toJson(), pos: op.pos + ins.str.length}, {});
   } else if (op instanceof OpStrDel) {
     if (!isPathEqual(ins.path, op.path)) return op;