Function open_retained_bracket 

pub fn open_retained_bracket(
    commands: &[Command],
    index: usize,
) -> Option<usize>

The outermost PushRetained bracket still open just before command index of commands — its command index — or None when no retained bracket is open there.

Pops are matched the way project_command_range matches them: a PopRetained closes the most recently opened retained bracket, whatever clips or layers were opened in between, and one with no retained bracket open is ignored. An index past the end asks about the end of the list, so a bracket never closed is open there. A range starting at index splits the bracket answered here: project_command_range demotes it to inline paint on both sides of the cut. Read-only and one pass over commands[..index].