Double cursor (ghost caret) appears in chat composer after two Shift+Enter line breaks

**Describe the bug**
When I type a few words in the chat composer and press Shift+Enter twice to add two line breaks, two cursors show up at the same time: the real one blinking in the right spot (after the second line break), and a leftover "ghost" cursor stuck one line above, where the cursor was after the first line break. It's only visual, but it's distracting and makes it hard to tell where text will actually go.

**Steps to reproduce**
1. Open any channel and click into the chat composer
2. Type a few characters or words on the first line
3. Press Shift+Enter twice
4. See two cursors: the real one after the second line break, and a ghost one on the line above it

**Expected behavior**
Just one cursor, blinking where I'm actually typing (the line after the second line break).

**Version and platform**
Find your version at the bottom of the Settings sidebar. Write "unknown" if you can't determine it.
- Buzz version: 0.4.26
- OS: macOS

**Logs / additional context**
Screenshot attached below. It happens when there's text on the first line followed by two Shift+Enters.