[FanWu-ai]

NestedCompleter uses whitespace-aware splitting and trimming, but only enters its nested-routing branch when it finds an ASCII space. show\t and show\n therefore suggest root commands instead of child commands; partial nested words and deeper levels fail too.

Recognize whitespace with str.isspace() in that guard, leaving key matching, child completers and cursor offsets unchanged. Add 29 cases for ASCII/Unicode separators, leading whitespace, partial words, cursor suffixes, unknown/terminal commands and multiple levels.

This is independent of #2120's case-insensitive key lookup, which leaves the separator guard unchanged. All new routing examples use exact-case command names.

Validation on Linux / Python 3.12.14:

• Fresh pristine complete suite: 156 passed; final complete suite: 185 passed, no skips. Twenty-one regressions fail on unchanged source, with eight controls passing.
• Ruff/format/typos, all three strict Mypy platform checks, all twelve configured pinned hooks, source/wheel builds and strict Twine metadata checks pass. Platform-aware type checks are not native OS test runs.
• Three real PromptSession Tab-completion flows pass with tab, newline and em-space separators.
• Independent automated review reran the full suite/static gates and checked 1,798 additional cases spanning all 29 Python whitespace characters and cursor offsets.

Other interpreters/native OS and hosted CI results are not claimed. Prepared and tested with OpenAI Codex assistance; no human review is claimed.