Line breaks render as multiple blank lines in chat composer/message bubble (worsens with paste round-trips)

Environment: macOS (Darwin 25.5.0), Claude Code chat window

Steps to reproduce:

1. Type a short multi-line message using Enter/Shift+Enter between lines.
2. Send it.
3. Observe the rendered message bubble.

Expected: One line break between lines, as typed.

Observed: Each line break renders as ~4-5 blank lines. Effect compounds further when copying content out of the composer and pasting it back in (roughly doubles per round-trip) — reproduces via keyboard paste, Cmd+Shift+V paste-and-match-style, and right-click paste alike, so it isn't a rich-text-formatting issue.