Environment

• bsk CLI/daemon: 0.3.2 and 0.3.3 (both reproduce; daemon version is not a factor)
• Browser extension: 0.3.3 (protocol 1.3), Chrome 155.0.0.0
• OS: Windows 11 x64, visible Agent Window started with --no-focus, BSK_AUTO_START=0
• Repro site: https://claude.ai/new chat input ([data-testid="chat-input"], a ProseMirror contenteditable)

Summary

With extension 0.3.3, bsk fill into the ProseMirror editor aborts with exit 3
fill target changed or lost focus while positioning the caret
when either:

1. the value is long — 453 chars succeeds, 980 and 1037 chars fail (5/5 attempts, across daemon 0.3.2 and 0.3.3), or
2. the editor already contains text — appending even 6 chars to a verified 453-char draft fails; every fill into non-empty PM content fails.

The same operations succeed on plain controls in the same session/tab:

Additional observations

• The failed operations complete in ≤36 ms per the debug operation timeline
  (debug operations / debug operation <id>), error message is
  fill target changed or lost focus while positioning the caret; the page records
  zero console errors and identical before/after states.
• Retries after the page has fully settled, and a real click into the editor
  beforehand, do not prevent the failure.
• Default fill (with its clear pass) always aborts at the same caret step on this
  editor — and its clear pass does wipe the field first, which makes it destructive
  on failure.
• Behavior changed between extension versions: on 0.3.2, a 1070-char --no-clear
  fill did insert the exact text (verified character-for-character via
  textContent) but returned exit 3 fill_value_mismatch — a read-back race against
  the page's async view update. On 0.3.3 short-value verification is clean, but
  long-value insertion and any append now hit the caret abort instead.

Workaround we use in production

User-parity clipboard insertion, which works for full-length values on the same editor:

bsk click @<editor-ref> --session ID
tr -d '\r\n' < message.txt | clip.exe        # or any OS clipboard setter
bsk press Ctrl+V --session ID
# verify via read-only textContent == intended text, then click the Send button

Resetting a draft also works via native keys: click → press Ctrl+A →
press Backspace → verify empty.

Expected behavior

• fill should succeed for long values on ProseMirror contenteditables, and should
  not treat ProseMirror's silent selection synchronization as "focus lost".
• Appending to a non-empty PM editor should not trip the caret/focus check.
• If the caret-positioning verification is intentionally strict, consider a graceful
  fallback (e.g. clipboard-paste insertion) or an explicit --paste flag instead of
  an abort that leaves the caller with no insertion path for long values.

Related closed PRs that touched fill behavior: #301, #488.