Fix cursor misalignment when scrolling past wrapped lines

[hgwr]

Summary

• keep cursor position consistent when scrolling past long wrapped lines
• add utility helper for calculating wrapped line height

Testing

• cargo build --verbose
• cargo test --verbose
• pytest -n auto e2e --verbose

---

https://chatgpt.com/codex/tasks/task_e_6896e3e66cac832f8f75544245ad8d3f

[gemini-code-assist]

Summary of Changes

Hello @hgwr, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

I've addressed a bug where the cursor position would become misaligned when scrolling through text that wraps across multiple lines in the editor. My changes ensure that the cursor's screen position is accurately maintained by correctly calculating the height of wrapped lines and adjusting the cursor's screen position accordingly during vertical scrolling.

Highlights

• Cursor Position Consistency: Ensured the cursor's screen position remains consistent and accurate, especially when scrolling past long, wrapped lines in the editor.
• Line Height Calculation: Introduced a new utility function get_line_height to precisely calculate the number of screen rows a given text line occupies, taking into account terminal width and character widths for proper line wrapping.
• Scrolling Logic Enhancement: Updated the NextLine command's scrolling mechanism to correctly account for the screen real estate occupied by wrapped lines, preventing cursor misalignment during vertical navigation.

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[gemini-code-assist]

Code Review

This pull request aims to fix cursor misalignment when scrolling past wrapped lines by introducing a helper function get_line_height and using it to correctly calculate screen line changes. This is a good approach.

My review has identified a critical flaw in the implementation of get_line_height which would lead to incorrect calculations and further bugs. I've also suggested a minor simplification for better code clarity.

Additionally, I noticed that the old logic for calculating line height still exists in NextLine::execute. It would be beneficial to replace it with a call to the new get_line_height helper to improve maintainability and ensure consistency.

[gemini-code-assist]

The current logic for calculating line height is flawed. It incorrectly increments the line count when the accumulated width is exactly equal to the terminal width, which leads to an extra line being counted for lines that perfectly fit the width. For example, for a line "abc" and width 3, it would return 2 instead of 1.

This can cause significant cursor misalignments, which is what this PR aims to fix. Here is a corrected implementation that properly handles line wrapping.

Suggested change

	let mut w = 0usize;
	let mut h = 1usize;
	for c in line.chars() {
	w += get_char_width(c) as usize;
	if w >= width as usize {
	w = 0;
	h += 1;
	}
	}
	h
	let mut w = 0usize;
	let mut h = 1usize;
	let width = width as usize;
	for c in line.chars() {
	let char_w = get_char_width(c) as usize;
	if w + char_w > width {
	h += 1;
	w = char_w;
	} else {
	w += char_w;
	}
	}
	h

[gemini-code-assist]

This if expression can be simplified by using saturating_sub, which is more idiomatic in Rust for this kind of calculation. It subtracts two numbers, returning 0 if underflow would occur, which is exactly the logic implemented here.

Suggested change

	let max_window_start =
	if editor.buffer.lines.len() > editor.content_height() as usize {
	editor.buffer.lines.len() - editor.content_height() as usize
	} else {
	0
	};
	let max_window_start = editor.buffer.lines.len().saturating_sub(editor.content_height() as usize);