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.h b/src/buffer.h
index 09daa29992a..c2dca0a10c1 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -144,6 +144,9 @@ enum { BEG = 1, BEG_BYTE = BEG };
 #define BUF_UNCHANGED_MODIFIED(buf) \
   ((buf)->text->unchanged_modified)
 
+#define BUF_UNCHANGED_SIZE(buf) \
+  ((buf)->text->unchanged_size)
+
 #define BUF_OVERLAY_UNCHANGED_MODIFIED(buf) \
   ((buf)->text->overlay_unchanged_modified)
 #define BUF_BEG_UNCHANGED(buf) ((buf)->text->beg_unchanged)
@@ -267,6 +270,9 @@ struct buffer_text
        end_unchanged contain no useful information.  */
     modiff_count overlay_unchanged_modified;
 
+    /* Buffer size as of last redisplay that finished.  */
+    ptrdiff_t unchanged_size;
+
     /* Properties of this buffer's text.  */
     INTERVAL intervals;