Add with_persistent_menus option

[maxim-uvarov]

Disclosure: the code was written by an AI agentunder my direction; I can't review Rust line-by-line myself. What was verified: the full reedline test suite plus a new 4-case test matrix (quick × persistent), cargo fmt --check / clippy with CI flags, and manual behavior checks in a live nushell build with both flag values. I've wanted this behavior for a long time and drove the design decisions. Please review accordingly.

Summary

When a menu is active and the user erases the characters they typed to filter it, the engine dismisses the menu. Two hardcoded paths in engine.rs do this: with quick completions on, any Backspace/BackspaceWord/MoveToLineStart deactivates the menu; and regardless of quick completions, any edit that leaves the line buffer empty deactivates it. Neither was configurable.

Erasing the filter is a natural part of browsing completions — narrowing too far and backing up shouldn't throw the menu away, especially since activating a menu on an empty buffer is already valid and shows all values.

Public API change: a new builder option, Reedline::with_persistent_menus(bool), default false. With the default, observable behavior is unchanged. With true, edits refilter an active menu instead of dismissing it; the menu still closes on Esc, Ctrl-C, or when a value is accepted.

Before

With a menu open, erasing input dismisses it:

• quick completions on: the first backspace closes the menu;
• quick completions off: the menu survives backspaces until an edit empties the line buffer, then closes.

After

Default (with_persistent_menus(false)): same as before.

with_persistent_menus(true): erasing characters (or emptying the line) sends MenuEvent::Edit like any other edit, so the menu stays open and refilters — down to the full unfiltered list on an empty buffer.

Additional notes

A follow-up nushell PR wires this as $env.config.completions.persistent_menus once a reedline release includes it.

Covered by a new rstest case matrix (quick × persistent) in src/engine.rs.

[fdncred]

I'd like to see how this work in an example so I can try it out before landing it.

[kronberger-droid]

Code looks good, not much to say since it plugs into the existing behavior.
If the example shows the expected behavoir its fine to land from my side.

[maxim-uvarov]

@fdncred @kronberger-droid below is the video demo. And nushell/nushell#18573 is the next change that is needed to use the reedline feature in nushell

Screen.Recording.2026-07-11.at.16.33.55.mov

[fdncred]

@maxim-uvarov This is fine but I'd like to see an example in the examples folder that demonstrates this reedline feature without nushell. I want to be able to run it myself without nushell to see how it works. Just look at the other examples as a guide on how to create one.

[maxim-uvarov]

@fdncred, this?
Added examples/persistent_menus.rs — runnable standalone:

cargo run --example persistent_menus

Type he, press Tab, then erase with Backspace past the start — the menu stays open and
refilters back up to the full list. Change with_persistent_menus(true) to false in the
example to compare with the current behavior (first Backspace dismisses the menu, since
quick completions are on).

Note: the example uses a small inline prefix completer instead of DefaultCompleter,
because DefaultCompleter returns no suggestions for empty input — the kept-open menu would
be invisible exactly at the empty-line moment the example demonstrates.

Also extended the test matrix to cover the other two commands in the quick-dismissal arm
(BackspaceWord, non-selecting MoveToLineStart).

[fdncred]

yup, that's what i meant. very cool. thanks!

[fdncred]

I'll let @kronberger-droid try it before we land it incase he sees something I missed.