Multi-line comment highlighting breaks after row deletion

### How to reproduce

Open a `.c` file with 4 lines:

```
int a;
int b;
int c;
int d;
```

1. Move cursor to start of line 2 (`int b;`), press Backspace — merges it into line 1
2. Move cursor to end of line 2 (`int c;`), type `/*`
3. Line 3 (`int d;`) should turn comment-colored but stays normal

For a control test, open the same file fresh (skip step 1), type `/*` at the end of any line — the line below immediately turns comment-colored.