Function display_col_to_source_byte 

pub fn display_col_to_source_byte(
    col: u32,
    max_source_byte: u32,
    inlay_offsets: &[(u32, u32)],
    conceals: &[ConcealRange],
) -> u32

Map a display column back to the source position under it — the inverse of source_byte_to_display_col, and what a mouse click needs.

max_source_byte is the source line’s length in the same already-char-resolved space the forward map uses (see the byte-vs-char note on crate::row::CellRow::byte_to_combined_col): the caller resolves char ↔ byte, this only undoes the column arithmetic. The result is clamped to [0, max_source_byte], so a click past the end of a line lands on its end rather than failing.

Derived from the forward map, not re-derived

This binary-searches source_byte_to_display_col for the largest source position whose column is still <= col, rather than running the inlay and conceal arithmetic backwards. That costs O(log line_len) forward evaluations — nothing, on a path driven by a human’s hand — and buys the one property that matters: the inverse cannot disagree with the forward map, because it is the forward map. This module exists because three copies of the forward arithmetic drifted; a hand-written inverse would be a fourth copy with the same failure mode and a worse symptom, since a click that lands one column off reads as a shaping bug rather than a missing conceal rule.

The search is sound because the forward map is monotonic non-decreasing: each source position adds one column plus any inlay spliced ahead of it, and a position inside a hidden range adds zero.

Where a click inside hidden or virtual text lands

On a concealed range, the first VISIBLE position at that column. Every position in [start, end] maps to the range’s start column, so the largest of them wins — which is the position just past the hidden text, i.e. the character actually drawn there. That is the right answer for a click and the mirror image of the forward map’s clamp, which sends a hidden position back to the range’s start column.

On inlay text, the source position before the splice. Inlay columns have no source position of their own; the one before them is the only truthful answer, and it is where the caret would already be drawn.