docs: document mobile autocomplete-disable fix

Explains why setting autocomplete/autocorrect/autocapitalize/spellcheck
attributes on xterm.js's hidden helper textarea is needed (and why a
MutationObserver) for mobile keyboards to stop corrupting per-keystroke
input. Still load-bearing alongside the new compose bar — covers chord
and arrow keys typed outside the compose input.

AUTOCOMPLETE_FIX.md

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+# Mobile Autocomplete Fix
+
+**Date:** 2026-03-28
+**File changed:** `static/toolbar.js`
+
+## The Problem
+
+On mobile, typing in the terminal caused glitchy re-typing of characters. For example, typing `git sta` would suddenly delete and re-insert text, producing garbled input like `git stagit sta`.
+
+## Root Cause
+
+xterm.js captures keyboard input through a hidden `<textarea>` element (class: `.xterm-helper-textarea`). Mobile browsers (iOS Safari, Android Chrome/Gboard) treat this like any other text field and activate autocomplete, autocorrect, autocapitalize, and spellcheck.
+
+When the keyboard's autocomplete activates, it:
+
+1. Deletes the characters you already typed (sends backspace events)
+2. Re-inserts the "corrected" or "completed" word
+
+xterm.js interprets this as real input — backspaces erase terminal content, then the replacement text gets typed out. The result is duplicated or garbled text visible on screen.
+
+## The Fix
+
+A `disableMobileAutocomplete()` function finds xterm's hidden textarea and sets four attributes that suppress mobile keyboard text assistance:
+
+```javascript
+ta.setAttribute('autocomplete','off');    // Chrome, general
+ta.setAttribute('autocorrect','off');     // Safari
+ta.setAttribute('autocapitalize','off');  // iOS (prevents auto-capitalizing first letter)
+ta.setAttribute('spellcheck','false');    // All browsers (removes red underlines + suggestions)
+```
+
+All four are needed because different browsers respect different attributes.
+
+## Why a MutationObserver
+
+The hidden textarea doesn't exist at page load — xterm.js creates it dynamically when the terminal mounts. The existing `MutationObserver` (which already handles mobile toolbar resizing) now also calls `disableMobileAutocomplete()` on every DOM change. A `data-acOff` guard ensures the attributes are only set once.
+
+## Nuclear Option (if still broken)
+
+Some aggressive keyboards (Samsung Keyboard, certain Gboard versions) may ignore these attributes entirely. If that happens, setting `inputMode="none"` on the textarea would fully disable the soft keyboard — but that requires adding a separate on-screen keyboard or toggle, since users would lose the ability to type normally.