Improve the heuristic for long lines detection.

* src/buffer.h (struct buffer_text): New 'unchanged_size' field.
(BUF_UNCHANGED_SIZE): New macro to access the field.

* src/buffer.c (Fget_buffer_create): Initialize the field.
(Fbuffer_swap_text): Handle it.

* src/xdisp.c (mark_window_display_accurate_1): Set the field.
(redisplay_window): Use the field for long lines detection.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/buffer.c b/src/buffer.c
index a777668e44b..9a463363b93 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -599,6 +599,7 @@ even if it is dead.  The return value is never nil.  */)
   BUF_OVERLAY_UNCHANGED_MODIFIED (b) = 1;
   BUF_END_UNCHANGED (b) = 0;
   BUF_BEG_UNCHANGED (b) = 0;
+  BUF_UNCHANGED_SIZE (b) = 0;
   *(BUF_GPT_ADDR (b)) = *(BUF_Z_ADDR (b)) = 0; /* Put an anchor '\0'.  */
   b->text->inhibit_shrinking = false;
   b->text->redisplay = false;
@@ -2477,6 +2478,11 @@ results, see Info node `(elisp)Swapping Text'.  */)
   other_buffer->text->beg_unchanged = other_buffer->text->gpt;
   other_buffer->text->end_unchanged = other_buffer->text->gpt;
   {
+    ptrdiff_t tmp = current_buffer->text->unchanged_size;
+    current_buffer->text->unchanged_size = other_buffer->text->unchanged_size;
+    other_buffer->text->unchanged_size = tmp;
+  }
+  {
     struct Lisp_Marker *m;
     for (m = BUF_MARKERS (current_buffer); m; m = m->next)
       if (m->buffer == other_buffer)