Fix scrolling after wrapped lines

[hgwr]

Summary

• adjust j movement so leaving a wrapped line doesn't scroll the window unexpectedly

Testing

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

---

https://chatgpt.com/codex/tasks/task_e_6896df2888f8832fa1d7cc80714f82e5

[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 an issue where moving the cursor down (j command) in the editor would cause unexpected screen scrolling when navigating through lines that wrap. My changes refine the NextLine command's behavior to accurately account for wrapped lines, ensuring that the cursor moves smoothly to the next logical line without causing the display to jump. This significantly improves the user experience when editing text with word wrapping enabled.

Highlights

• Improved Cursor Movement for Wrapped Lines: The core change is within the NextLine command, which is typically bound to the j key for moving the cursor down. The previous implementation could cause the screen to jump unexpectedly when moving from a wrapped line to the next logical line.
• Precise Calculation of Remaining Wrapped Lines: The updated logic now calculates how many display lines remain on the current logical line from the cursor's current position. This calculation considers the terminal's width and the width of individual characters.
• Smarter Screen and Window Scrolling: Based on the calculated remaining lines, the code intelligently adjusts both the cursor's screen position and the editor's window position in the buffer. This ensures that the screen scrolls only when necessary and by the correct amount, preventing jarring visual jumps.

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[gemini-code-assist]

Code Review

The pull request adjusts the 'j' key movement to correctly handle scrolling with wrapped lines. The logic for calculating line wrapping information is duplicated, which can be refactored for better maintainability and slight performance improvement. Otherwise, the change correctly addresses the issue of unexpected scrolling.

[gemini-code-assist]

The logic to calculate cursor_row_in_line and line_height involves iterating over the characters of current_line twice. This can be optimized by combining these two loops into a single pass over the line's characters. This refactoring will improve code readability and reduce redundancy.

            // Calculate both cursor_row_in_line and total line_height in one pass
            let mut width = 0usize;
            let mut cursor_row_in_line = 0usize;
            let mut line_height = 1usize;
            let cursor_col = editor.cursor_position_in_buffer.col;

            for (i, c) in current_line.chars().enumerate() {
                if i == cursor_col {
                    cursor_row_in_line = line_height - 1;
                }

                width += get_char_width(c) as usize;
                if width >= editor.terminal_size.width as usize {
                    width = 0;
                    line_height += 1;
                }
            }
            if cursor_col >= current_line.chars().count() {
                cursor_row_in_line = line_height - 1;
            }