Slow prompt display (~2s) with terminals that don't respond to cursor position queries

[po-nuvai]

Description

Some terminal emulators don't properly respond to CSI 6n (Device Status Report) cursor position queries, causing a ~2 second delay on every prompt display.

This was reported in nushell/nushell#17210 by a user experiencing 2-second delays when using Termius on Android to SSH into machines with nushell as the login shell.

Root Cause

In src/painting/painter.rs:174, initialize_prompt_position() calls:

let prompt_selector = select_prompt_row(suspended_state, cursor::position()?);

This cursor::position() (from crossterm) sends a CSI 6n escape sequence and waits for the terminal to respond with the cursor position. If the terminal doesn't respond, crossterm times out after ~2 seconds.

Since initialize_prompt_position() is called at the start of every read_line(), this delay happens on every prompt display, not just startup.

Affected Terminals

• Termius on Android - doesn't respond to CSI 6n queries (the reported case)
• Potentially other mobile/embedded terminal emulators

Note: Termius on Linux works fine, as does OpenSSH from Termux on the same Android device.

Suggested Solutions

1. Shorter timeout with fallback: Reduce the timeout for cursor::position() and fall back to assuming position (0, 0) or tracking position internally when the query fails/times out

2. Configuration option: Add a way to disable cursor position queries entirely for problematic terminals (e.g., Reedline::disable_cursor_position_query() or similar)

3. Environment variable: Check for something like REEDLINE_NO_CURSOR_QUERY=1 to skip the query

4. Lazy/cached querying: Only query cursor position when absolutely necessary, and cache the result

Reproduction

1. Use Termius Android app to SSH into a machine with nushell as the login shell
2. Observe ~2 second delay after every command before the prompt appears

The delay shows up in nushell's perf logging as:

setup reedline took 2.001272815s

References

• Original issue: Slow startup - "setup reedline" - using Termius on Android nushell#17210
• Crossterm cursor position implementation uses a blocking read with timeout
• Similar issues exist in other shells/editors with terminals that don't respond to DSR queries

[fdncred]

I wonder if we implement that TERM=dumb functionality I posted here #1009 if it would/could fix this problem. If TERM=dumb there's no reason to call ansi escapes at all.