Hit-Testing#

#Overview

Hit-testing converts a pointer position (in viewport coordinates) to the cell, column header, or row header under the cursor. It is the foundation for click selection, hover, editing, and context menus.

#Functions

#hit_test(vx, vy, model, scroll_x, scroll_y) → Option<CellCoord>

Resolves a data cell from viewport coordinates:

1. Gutter check — if vx < row_number_width, returns None
2. Header check — if vy < header_height, returns None
3. Column lookup — uses ColumnOffsets for O(1) lookup
4. Row lookup — O(1) thanks to uniform row height

Returns None if the pointer is below the last row or right of the last column.

#hit_test_col_header(vx, vy, model, scroll_x) → Option<usize>

Returns the column index when clicking a column header. Used for:

• Sort toggle
• Column selection
• Column resize detection

#hit_test_row_header(vx, vy, model, scroll_y) → Option<u64>

Returns the row index when clicking the row number gutter. Used for:

• Row selection
• Row selection extension (drag)

#Pinned columns

Hit-testing accounts for pinned columns:

if vx_data < pinned_width:
    abs_x = vx_data          // pinned: no scroll offset
else:
    abs_x = vx_data + scroll_x  // scrollable: add scroll

This ensures clicks on pinned columns resolve correctly regardless of horizontal scroll position.

#Precision at extreme scroll positions

For very large datasets (billions of rows), naive (vy + scroll_y - header_height) / row_height loses f64 precision because it subtracts two large numbers.

The hit-testing code uses a precision-preserving decomposition:

let sy_content = scroll_y - header_height;
let first_row = (sy_content / row_height) as u64;
let frac = sy_content % row_height;  // sub-row offset
let offset = ((vy + frac) / row_height) as u64;
let row = first_row + offset;

This keeps intermediate values small, preserving precision even at row indices near the u64 limit.

#Performance

• Column lookup: O(1) via precomputed ColumnOffsets
• Row lookup: O(1) with uniform row height
• Total: O(1) per hit-test call