Draft.js: type_text / insert_text append must not double or replace existing content (clear_first:false)

[Trace824]

Summary

On X Draft.js compose, type_text / insert_text with clear_first: false (append) can duplicate text (body + body) or wipe/replace the existing draft. Append-without-wipe is the hard invariant for multi-step compose (body, then URL line).

Repro

• Context: x.com compose, Draft.js [data-testid=tweetTextarea_0], existing draft text already present.
• Steps:
  1. Insert body text once (CDP insert_text or selector type_text).
  2. Append a second string with clear_first: false / omit clear.
  3. Read draft contents before send.
• Observed: doubled text (e.g. body repeated) or second call replaces first; clear_first / select-all paths also flake.
• Partial mitigation on branch: draft_fast_path caret-at-end + clearFirst only when explicitly true (~actions.js L2967–3005); quirks remain under Draft.js / CDP mix.

Code pointers (verified against /workspace/FSB)

• extension/content/actions.js — draft_fast_path clear/append (~L2967–3008); placeCaretAtEnd (~L3492+); clear_first / clearFirst handling (~L3473–3490, ~L3729–3730); append-safe verify comment (~L3667).
• extension/background.js — cdpInsertText clearFirst (~L19131+, ~L20055–20093) via Input.insertText.
• extension/ai/tool-definitions.js — insert_text / type_text schemas.

Proposed fix

Single root cause: append path must never select-all or replace Draft editor state when clear_first is false. Unify selector draft_fast_path and CDP cdpInsertText so both (1) place caret at end, (2) insert once, (3) verify post-text contains prior content + new suffix exactly once. Prefer one insertion API end-to-end for compose (see related paste issue).

Acceptance / regression

• Given draft Hello, append \nhttps://example.com/ → draft is exactly body + newline + URL once.
• Never doubles; never clears unless clear_first: true.
• Automated regression for clear_first false/true on a Draft-like contenteditable fixture.

Owner

extension

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Priority: P0 · Owner: extension

[LakshmanTurlapati]

Fixed in #143: type_text takes clear_first: false to append, and multi-line text is verified once against the rendered text, so it is no longer doubled or replaced.