handle half-complete markdown formatting markers

[jinghaihan]

Bug Description

In intermediate states, there can be a brief moment where a strong token is parsed into the following AST structure. This causes visual flickering on the page.

The scenarios I have identified so far include strong and strikethrough.

This is **bold text*

{
    "type": "root",
    "children": [
        {
            "type": "paragraph",
            "children": [
                {
                    "type": "text",
                    "value": "This is *",
                    "position": {
                        "start": {
                            "line": 1,
                            "column": 1,
                            "offset": 0
                        },
                        "end": {
                            "line": 1,
                            "column": 10,
                            "offset": 9
                        }
                    }
                },
                {
                    "type": "emphasis",
                    "children": [
                        {
                            "type": "text",
                            "value": "bold text",
                            "position": {
                                "start": {
                                    "line": 1,
                                    "column": 11,
                                    "offset": 10
                                },
                                "end": {
                                    "line": 1,
                                    "column": 20,
                                    "offset": 19
                                }
                            }
                        }
                    ],
                    "position": {
                        "start": {
                            "line": 1,
                            "column": 10,
                            "offset": 9
                        },
                        "end": {
                            "line": 1,
                            "column": 21,
                            "offset": 20
                        }
                    }
                }
            ],
            "position": {
                "start": {
                    "line": 1,
                    "column": 1,
                    "offset": 0
                },
                "end": {
                    "line": 1,
                    "column": 21,
                    "offset": 20
                }
            }
        }
    ],
    "position": {
        "start": {
            "line": 1,
            "column": 1,
            "offset": 0
        },
        "end": {
            "line": 1,
            "column": 21,
            "offset": 20
        }
    }
}

I’m working on a similar approach myself, and I’d be glad if this could be useful for Streamdown:
strong

From my perspective, I’d really prefer to see these requirements handled as much as possible within remend, instead of everyone maintaining their own implementations.

Steps to Reproduce

import { describe, expect, it } from "vitest";
import remend from "../src";

describe("half complete", () => {
  it("should handle incomplete bold", () => {
    expect(remend("This is **bold text*")).toBe("This is **bold text**");
  });

  it("should handle incomplete bold", () => {
    expect(remend("This is __bold text_")).toBe("This is __bold text__");
  });

  it("should handle incomplete strikethrough", () => {
    expect(remend("This is ~~bold text~")).toBe("This is ~~bold text~~");
  });
});

Expected Behavior

Expected: "This is bold text"
Expected: "This is bold text"
Expected: "This is bold text"

Actual Behavior

Received: "This is *bold text"
Received: "This is _bold text"
Received: "This is ~~bold text~"

Streamdown Version

latest

React Version

latest

Node.js Version

v24.12.0

Browser(s)

No response

Operating System

None

Additional Context

No response