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973f0f6134
Make this pass on both macOS and Linux.
This was an obnoxious and uninteresting test to debug and so I used
claude code. It insists this is due to differences in pty handling between
macOS and Linux. Specifically it writes:
The test was failing inconsistently because macOS and Linux have different
PTY scrollback behavior after rendering prompts with right-prompts.
Root cause: After fish writes a right-prompt to the rightmost column,
different PTY drivers position the cursor differently:
- macOS PTY: Cursor wraps to next line, creating a blank line
- Linux PTY: Cursor stays on same line, no blank line
This is OS kernel-level PTY driver behavior, not terminal emulator behavior.
Fix: Instead of hardcoding platform-specific offsets, detect the actual
terminal behavior by probing the output:
1. Capture with -S -12 and check if the first line is blank
2. If blank (macOS behavior), use -S -13 to go back one more line
3. If not blank (Linux behavior), use -S -12
Also split the C-l (clear-screen) command into its own send-keys call
with tmux-sleep after it, ensuring the screen clears before new output
appears. This improves test stability on both platforms.
The solution is platform-independent and adapts to actual terminal
behavior rather than making assumptions based on OS.