Buffer: change clamp logic to preserve ordering

clamp could change ordering between a coordinate past the end.

Say in a buffer with 1 line of 2 char:
{0, 1} was clamped to {0, 1}
{1, 0} was clamped to {0, 0}

That was reversing their ordering, and might be the root cause
of the bug lurking in undo range computation.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/buffer.cc b/src/buffer.cc
index 9650ee20..0ffdb9ae 100644
--- a/src/buffer.cc
+++ b/src/buffer.cc
@@ -172,7 +172,9 @@ BufferIterator Buffer::iterator_at(BufferCoord coord) const
 
 BufferCoord Buffer::clamp(BufferCoord coord) const
 {
-    coord.line = Kakoune::clamp(coord.line, 0_line, line_count() - 1);
+    if (coord > back_coord())
+        coord = back_coord();
+    kak_assert(coord.line >= 0 and coord.line < line_count());
     ByteCount max_col = std::max(0_byte, m_lines[coord.line].length() - 1);
     coord.column = Kakoune::clamp(coord.column, 0_byte, max_col);
     return coord;