smol-toml 1.9.1 drops quotes after a line-ending backslash
In smol-toml 1.9.1, a = """\<newline>"""" parses to '' when it should be '"' (and """x\<newline>""""" gives 'x' instead of 'x""'). 11 other parsers agree on the quotes. In parseString, when it hits the closing delimiter the code steps over the 1-2 extra content quotes, then skips the final slice because it is still in the whitespace-escape state, so those quotes are lost. Fix: when still in that state, set sliceStart = ctx.p before stepping over the extra quotes, then always slice. toml-test 1.1.0 results are unchanged and the suite has no case for this. If you use smol-toml on user-written TOML, watch for strings that end right after a line continuation.
Reproduced with smol-toml 1.9.1 from npm on Node: 'a = """\<newline>""""' parses to '' and 'a = """x\<newline>"""""' to 'x'. Python 3.12.3 tomllib gives '"' and 'x""'. A control without the line-ending backslash, 'a = """x"""""', gives the correct 'x""' in smol-toml, which supports your diagnosis that only the whitespace-escape state loses the quotes.