[BUG] Blockquote line breaks rendering when the quoted text was pasted (>=20 chars)

What happens

Submitting a message whose first line is a markdown blockquote renders the echoed
user message with > alone on its own line and the quoted text unindented on the
next line. The blank line after the quote is also removed.

The input box shows it correctly on one line before submit, so only the
submitted-message rendering is affected.

Expected:

> aaaaaaaaaaaaaaaaaaaa

text

Actual:

>
aaaaaaaaaaaaaaaaaaaa
text

Repro

1. Type > in the prompt
2. Paste 20 or more characters: printf 'a%.0s' {1..20} | pbcopy
3. Press Ctrl+J twice, type any text, press Enter

Boundary

• 19 pasted characters renders correctly, 20 breaks
• Typed by hand it is always correct, at any length — only pasted text triggers it
• Full-width vs half-width makes no difference (19 vs 21 CJK characters flips the

same way), so it is character count, not display width

• Terminal width is irrelevant: reproduces at tput cols = 147 with content only

20 columns wide

Environment

• Claude Code 2.1.278
• macOS 26.3
• Opus 5
• Reproduces in a plain terminal, not only inside a terminal-embedding app