Problem

In #2810, Buffer::set_stringn panicked when called with coordinates outside the buffer's area (e.g., in a 57x1 terminal split inside tmux where a multi-row layout writes to row y = 1).

Buffer::set_stringn computed remaining_width from self.area.right().saturating_sub(x), but did not verify whether y fell within [area.top(), area.bottom()) or whether x was already outside the area. Writing to self[(x, y)] subsequently panicked in Buffer::index_of.

Solution

Add an early-return guard at the start of Buffer::set_stringn:

if y < self.area.top() || y >= self.area.bottom() || x >= self.area.right() {
    return (x, y);
}

If coordinates fall outside the buffer area, it safely early returns without mutating buffer contents or panicking, mirroring the safe behavior in Buffer::set_style.

Added unit test set_stringn_does_not_panic_when_out_of_area in ratatui-core/src/buffer/buffer.rs.

Fixes #2810.