fix(desktop): truncate large text paste to prevent UI freeze

[thedavidweng]

Problem

Pasting a large multi-line text (e.g. a full log file with thousands of lines) into the chat composer freezes the entire application. The UI becomes completely unresponsive and must be force-quit.

The root cause is that document.execCommand('insertText') creates one DOM node per line in the contentEditable div, followed by a synchronous recursive DOM walk (composerPlainText) and a React state update — all on the main thread. For thousands of lines, this blocks the UI thread long enough to trigger a freeze.

Fix

Add a MAX_PASTE_LENGTH (50,000 characters) hard limit in handlePaste. When the pasted text exceeds this threshold it is truncated before insertion and a warning notification is shown:

"Paste truncated — Text was truncated to 50,000 characters."

This keeps the DOM manageable and prevents the freeze while still allowing large pastes up to a generous limit (roughly 1,000–2,000 lines depending on line length).

Changes

• apps/desktop/src/app/chat/composer/index.tsx — add MAX_PASTE_LENGTH constant and truncation logic in handlePaste

Fixes #40147

[teknium1]

Thanks for the report and focused fix. This is an automated hermes-sweeper review: current main already resolves the large multiline-paste freeze without truncating user input.

• 0a865e5948cb836eba93620e365703ec38cf76e3 (fix(desktop): bypass Chromium editing pipeline for large paste & select-delete, fix(desktop): bypass Chromium editing pipeline for large paste & select-delete #45812) replaced the Chromium execCommand('insertText') path targeted by this PR.
• Current apps/desktop/src/app/chat/composer/index.tsx:359-361 prevents the native paste, inserts via insertPlainTextAtCaret, and coalesces the draft flush.
• apps/desktop/src/app/chat/composer/rich-editor.ts:147-173 implements that helper with a Range and text/<br> fragment, specifically avoiding Chromium's quadratic multiline editing path.
• Regression coverage is present in apps/desktop/src/app/chat/composer/rich-editor.test.ts:75-110.
• The linked issue [Bug]: Desktop app freezes when pasting large multi-line text into composer #40147 was already closed with the same verified implementation; the fix shipped in v2026.6.19.