clamp_selection

Return selection shifted past a diff row's gutter, if one is set.

Every form a selection can take over a single-line row covers the gutter unless its start is moved past it:

• Selection(None, None) — the row sits mid-selection. Textual extracts the row's full text, so the start must move to the gutter's end even though no endpoint lands here.
• Selection(None, end) — entered from above; same move, plus an end still inside the gutter means nothing selectable is covered, reported as None so the row drops out of the screen's selection map.
• Selection(start, None) / Selection(start, end) — pull any endpoint inside the gutter forward to its end; a range that then collapses (wholly gutter) is None.

An endpoint counts as "inside the gutter" only on the row's first visual line (y == 0). A row wrapped by Textual continues at column 0 of each following visual line, where the gutter no longer exists — an x there already indexes source text, so a continuation coordinate must pass through untouched or a drag starting on a continuation would skip its first gutter-width characters, and one ending within them would drop source the user visibly selected.