[AryanRG]

Fixes #1467.

In Vi mode, typing asdf, Escape, then u currently leaves asd; this restores the pre-3.0.4 result of an empty buffer. An earlier insert is preserved (x followed by a new asdf insert undoes back to x ). Emacs ControlUnderscore and repeated backspace grouping are covered as well.

Named commands became Binding objects in 3.0.4. Re-registering them through KeyBindings.add silently drops the explicit save_before callbacks already supplied by the built-in self-insert, delete and Emacs undo bindings. This change makes an explicit callable take effect, while omitted callbacks continue to inherit the source binding's callback.

This deliberately changes public API precedence: KeyBindings.add(..., save_before=f)(existing_binding) now uses f. Existing-Binding precedence was not documented; explicit-caller precedence is inferred from the parameter description, the built-in callers and historical behavior. Omitting save_before or passing None inherits the source callback. Plain-handler registration with an omitted or callable value keeps its existing behavior; explicit None now means omitted instead of failing when called. The public signature default changes to None. Falsy callable objects remain valid overrides. The Binding constructor and generic key_binding() decorator keep their existing defaults, and other binding metadata handling is unchanged.

If preserving explicit-callback precedence for custom Binding objects is preferred, an alternative is an unspecified-callback marker limited to named commands. This patch chooses the consistent public add behavior and documents it. It leaves the buffer undo/redo implementation unchanged; #2083 addresses separate redo invalidation behavior.

Validation:

• New regressions on the unmodified source: 9 failed, 42 passed across the focused files; with the fix: 51 passed.
• Full suite: 167 passed on Python 3.12.15 and 3.14.4.
• ruff check ., ruff format --check ., and typos . passed.
• mypy --strict src/ passed for win32, linux, and darwin (145 source files each).
• Historical 3.0.3 and current pipe-input probes confirm the Vi and Emacs grouping expectations.

AI assistance: AI agents implemented the change and performed separate automated plan and final reviews, including executable checks. Submitted as a draft for maintainer feedback on the API tradeoff; the issue has no maintainer statement endorsing this approach. Review may take time given the existing PR queue.