TUI: typing large amounts of text into the chat input causes severe lag

Description

Typing (or dictating) a large amount of text into the TUI chat input (the prompt box) makes the UI lag heavily and effectively stop responding as the buffer grows. Pasting large blobs is usually collapsed into a "[Pasted ~N lines]" summary, so this is most visible when typing or voice-dictating directly into the input.

Root cause found while profiling: mentionTriggerIndex runs on every keystroke (onContentChange → autocomplete onInput) and performs two full-buffer Intl.Segmenter passes per keystroke. Measured per-keystroke cost:

• 5 KB buffer: ~3.1 ms
• 20 KB: ~13 ms
• 100 KB: ~65 ms

The underlying opentui textarea is not the bottleneck (~0.2 ms/keystroke into a 20 KB buffer).

Waiting on PR #49634 which adds cheap raw-string gates so the display-aware scan only runs when an actual @ trigger needs resolving.

Plugins

None

OpenCode version

1.18.31

Steps to reproduce

1. Open the opencode TUI
2. Type or dictate a large amount of text into the chat input (tens of KB)
3. Observe keystroke latency climbing as the buffer grows until the UI appears frozen

Expected behavior

Keystroke latency stays roughly constant regardless of how much text is in the input box.

Operating System

Linux

Terminal

Any