feat(shell): add fish-style Ctrl-L to clear screen preserving scrollback

[jetjinser]

Related Issue

N/A (Minor feature addition matching fish shell behavior)

Description

Pressing Ctrl+L now pushes the current prompt content into the terminal's scrollback buffer (like fish's scrollback-push)

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any.
• I have added tests that prove my fix is effective or that my feature works.
• I have run make gen-changelog to update the changelog.
• I have run make gen-docs to update the user documentation.

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 3 additional findings in Devin Review.

[devin-ai-integration]

🟡 Incorrect scroll calculation under-pushes content into scrollback with multi-line input

The Ctrl+L handler computes scroll = max(0, height - used_lines) where used_lines is window.render_info.content_height — the line count of the input buffer content, not the total rendered height of the application. This means the number of newlines written decreases as the input grows, resulting in fewer rows being pushed into scrollback.

Detailed explanation and impact

content_height at prompt.py:719 returns ui_content.line_count, which is the number of lines in the text being edited in the input buffer. For a fish-style clear, the goal is to push the entire visible screen into scrollback by writing height newlines (one per terminal row).

With the current formula scroll = max(0, height - used_lines):

• Terminal height 40, empty input (1 line): scroll = 39 — pushes 39 of 40 rows ✓ (close enough)
• Terminal height 40, 10-line input: scroll = 30 — only pushes 30 rows, but the app renders ~12+ rows of content. The top ~10 rows of the old screen are not pushed into scrollback and are lost when renderer.clear() calls erase_screen().

The fix should be to write height newlines unconditionally (or at minimum, use the total rendered height of the application rather than just the input buffer content height):

scroll = output.get_size().rows
output.write("\n" * scroll)

Impact: With multi-line input, pressing Ctrl+L fails to preserve the top portion of the visible screen in scrollback history, contradicting the documented fish-style behavior.

Suggested change

	height = output.get_size().rows
	used_lines = window.render_info.content_height
	# calculate scroll to push content to top; guard against negative
	scroll = max(0, height - used_lines)
	height = output.get_size().rows
	output.write("\n" * height)
	output.flush()

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[jetjinser]

This code is written intentionally.

Subtracting used_lines is to prevent the current prompt from scrolling up, thus preventing duplicate prompts (both the scrolled-up prompt and the refreshed prompt).