@std/yaml: stringify() with condenseFlow wraps already quoted keys in a second pair of quotes #7362

What happens

With condenseFlow: true, stringify() wraps every flow mapping key in double quotes, even when the key was already emitted as a quoted scalar. Keys that need quoting (numeric strings, true, #, the empty string, strings with " or a newline) come out double wrapped. Parsing the output either returns a different key or throws.

import { parse, stringify } from "jsr:@std/yaml";

const options = { flowLevel: 0, condenseFlow: true };

stringify({ "1": 1 }, options);    // {"'1'":1}    parses back as { "'1'": 1 }
stringify({ "": 1 }, options);     // {"''":1}     parses back as { "''": 1 }
stringify({ 'a"b': 1 }, options);  // {"a"b":1}    parse() throws
stringify({ "a\nb": 1 }, options); // {""a\nb"":1} parse() throws
stringify({ "a\\nb": 1 }, options); // {"a\nb":1}  parses back with a real newline

Keys longer than 1024 characters also get the ? explicit key marker inside the quotes, so the parsed key starts with ? .

What should happen

The output should be valid YAML that parses back to the same object, for example {'1':1} and {"a\"b":1}.

Root cause

DumperState.stringifyFlowMapping() in yaml/_dumper_state.ts builds each pair as quote + keyPrefix + keyString + quote, where keyString is the already serialized key and may already be quoted. Plain keys are also wrapped without escaping, so a plain key containing \ or " changes meaning inside the double quotes.