[trissim]

P407: abstract ancestor capabilities

Fixes the Textual dependency of OpenHCSDev/toad#1115. SessionScope is an abstract DOMNode capability, not a concrete Widget constructor. The old type[QueryType] overload rejected both its Widget bound and its abstractness.

• src/textual/css/query.py:53: one unbound AncestorType; existing Widget-bound QueryType is unchanged.
• src/textual/dom.py:1820: replace the class-selector overload with TypeForm[AncestorType]; implementation returns DOMNode | AncestorType. String and expect_type overloads are unchanged.
• Decode the external selector once. CSS strings retain the existing DOMQuery matcher; nominal classes use isinstance. Both feed one ancestor walk and one unchanged expect_type filter. Reject non-class type forms explicitly at that boundary. No cast, ignore, Any, alternate API, or duplicated walk.
• Keep upstream Python 3.9 support: ordinary boundary decoding, not a match statement; metadata/ruff minimum unchanged.
• tests/test_query.py: one abstract-mixin test returning the exact existing parent instance. The abstractmethod-only mixin follows Textual's metaclass-compatible convention.

Verification at ad554b3

Background commands, existing dependencies, timeout 300, faulthandler_timeout=60:

pytest -q -n 0 -o faulthandler_timeout=60 tests/test_query.py::test_query_ancestor_abstract_mixin
1 passed in 0.36s

pytest -q -n 0 -o faulthandler_timeout=60 tests/test_query.py
17 passed in 0.44s

The second count includes the new case. No full suite or local ratchet.

A focused mypy caller using the real Toad SessionScope and assert_type(child.query_ancestor(SessionScope), SessionScope) passes without a cast/ignore. The whole dom.py check still reports 33 diagnostics outside query_ancestor; the two introduced implementation errors from the first annotation-only checkpoint are gone. Python ast.parse(feature_version=(3,9)) accepts the changed module. No installation claim.

Owner/consumer evidence

Existing refactor-audit Package/Repository inventory: 255 production modules and 484 test modules, zero parse omissions. It enumerates query declarations, imports, QueryType/AncestorType, DOMQuery._matches, of_type, and every syntactic query_ancestor consumer. Toad's separate complete Scope inventory is on Textualize#1115. Dynamic aliases are not claimed resolved by AST.

Search: rg -n 'AncestorType|def query_ancestor|TypeForm' src/textual/{css/query,dom}.py shows one AncestorType declaration and the replacement overload/implementation. The superseded constructor overload is deleted, not kept alongside the new one.

Assumed:

• Ancestor selectors are CSS strings or nominal runtime classes; arbitrary non-class TypeForm values are not a second selector language.
• Runtime nominal membership remains Python isinstance; CSS selection remains DOMQuery's existing matcher.

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