Word Wrap Scroll Correctness Fixes

March 6, 2026 · View on GitHub

Overview

Fixed multiple functions in the scroll/viewport pipeline that assumed uniform line heights, causing incorrect behavior when word wrap is enabled (where lines can have variable heights depending on how many visual rows they wrap into).

Key Files

src/editor/ferrite/view.rs — ViewState: scroll position, viewport, coordinate conversion
src/editor/widget.rs — EditorWidget: scroll sync, viewport restoration

Problem

When word wrap is enabled, a single logical line can span multiple visual rows with height greater than the base line_height. Several core functions ignored this and used line * line_height for all calculations, causing:

Click-to-place-cursor mapped to wrong lines
Go-to-line (Ctrl+G) didn't center correctly
"Ensure cursor visible" after typing miscalculated viewport bounds
Sync scrolling between split view panes was misaligned
Minimap position indicator was wrong
Scroll restoration after content sync used wrong positions

Fixes Applied

view.rs

Function	Before	After
`pixel_to_line()`	`pixel_y / line_height`	Uses `get_line_y_offset()` + `y_offset_to_line()` binary search when wrap active
`line_to_pixel()`	`line_diff * line_height`	Uses `get_line_y_offset(line) - viewport_top` when wrap active
`scroll_to_center_line()`	`viewport_height / line_height` for visible count	Uses actual pixel positions via `scroll_to_absolute()`
`is_line_visible()`	`first_visible + ceil(viewport/line_height)`	Compares `get_line_y_offset(line)` against viewport pixel bounds
`ensure_line_visible()`	Same uniform assumption	Uses `scroll_to_absolute()` with actual line bottom position

All fixes are guarded by self.is_wrap_enabled() && self.cumulative_heights.len() > 1, so the uniform-height fast path is unchanged for non-wrapped documents.

widget.rs

Location	Before	After
`tab.scroll_offset`	`first_visible * line_height`	`view.current_scroll_y()` (absolute pixel position)
`EditorOutput.scroll_offset`	`first_visible * line_height + scroll_offset_y`	Same absolute position
`pending_scroll_offset`	`offset / line_height` → `scroll_to_line()`	`scroll_to_absolute(offset, total_lines)`
`pending_sync_scroll_offset`	`offset / line_height` → `scroll_to_line()`	`scroll_to_absolute(offset, total_lines)`
Viewport restoration	`scroll_offset / line_height` → `scroll_to_line()`	`scroll_to_absolute(scroll_offset, total_lines)`

How It Works

ViewState maintains two parallel data structures for wrapped documents:

wrap_info: Vec<WrapInfo> — Per-line visual row count and pixel height
cumulative_heights: Vec<f32> — Prefix sum array where cumulative_heights[i] = total height of lines 0..i

These enable O(log N) binary search from y-offset to line number via y_offset_to_line(), and O(1) line-to-y-offset via get_line_y_offset().

The fixed functions use these structures when wrap is active, falling back to the line * line_height fast path when wrap is off or height data isn't yet available.

Task 40 — Performance optimization of the wrap scroll pipeline (incremental cache rebuild, O(1) LRU, O(log N) visual row mapping)
Task 41 — Profiling and verification of 60fps scroll performance