Improve detection of long lines.

* src/buffer.h (struct buffer_text): New field.
(BUF_CHARS_UNCHANGED_MODIFIED, CHARS_UNCHANGED_MODIFIED): New macros.

* src/buffer.c (Fget_buffer_create): Initialize the new field.

* src/xdisp.c (mark_window_display_accurate_1): Set the new field.
(redisplay_window): Use it, together with CHARS_MODIFF, instead of
MODIFF and UNCHANGED_MODIFIED to decide whether to check for long
lines.

* src/pdumper.c (dump_buffer): Dump the new field.

src/buffer.c

@@ -597,6 +597,7 @@ even if it is dead.  The return value is never nil.  */)
   set_buffer_intervals (b, NULL);
   BUF_UNCHANGED_MODIFIED (b) = 1;
   BUF_OVERLAY_UNCHANGED_MODIFIED (b) = 1;
+  BUF_CHARS_UNCHANGED_MODIFIED (b) = 1;
   BUF_END_UNCHANGED (b) = 0;
   BUF_BEG_UNCHANGED (b) = 0;
   *(BUF_GPT_ADDR (b)) = *(BUF_Z_ADDR (b)) = 0; /* Put an anchor '\0'.  */

src/buffer.h

@@ -149,12 +149,18 @@ enum { BEG = 1, BEG_BYTE = BEG };
 #define BUF_BEG_UNCHANGED(buf) ((buf)->text->beg_unchanged)
 #define BUF_END_UNCHANGED(buf) ((buf)->text->end_unchanged)
 
+#define BUF_CHARS_UNCHANGED_MODIFIED(buf) \
+  ((buf)->text->chars_unchanged_modified)
+
 #define UNCHANGED_MODIFIED \
   BUF_UNCHANGED_MODIFIED (current_buffer)
 #define OVERLAY_UNCHANGED_MODIFIED \
   BUF_OVERLAY_UNCHANGED_MODIFIED (current_buffer)
 #define BEG_UNCHANGED BUF_BEG_UNCHANGED (current_buffer)
 #define END_UNCHANGED BUF_END_UNCHANGED (current_buffer)
+
+#define CHARS_UNCHANGED_MODIFIED \
+  BUF_CHARS_UNCHANGED_MODIFIED (current_buffer)
 
 /* Functions to set PT in the current buffer, or another buffer.  */
 
@@ -268,6 +274,9 @@ struct buffer_text
        end_unchanged contain no useful information.  */
     modiff_count overlay_unchanged_modified;
 
+    /* CHARS_MODIFF as of last redisplay that finished.  */
+    modiff_count chars_unchanged_modified;
+
     /* Properties of this buffer's text.  */
     INTERVAL intervals;
 

src/pdumper.c

@@ -2764,6 +2764,7 @@ dump_buffer (struct dump_context *ctx, const struct buffer *in_buffer)
       DUMP_FIELD_COPY (out, buffer, own_text.end_unchanged);
       DUMP_FIELD_COPY (out, buffer, own_text.unchanged_modified);
       DUMP_FIELD_COPY (out, buffer, own_text.overlay_unchanged_modified);
+      DUMP_FIELD_COPY (out, buffer, own_text.chars_unchanged_modified);
       if (buffer->own_text.intervals)
         dump_field_fixup_later (ctx, out, buffer, &buffer->own_text.intervals);
       dump_field_lv_rawptr (ctx, out, buffer, &buffer->own_text.markers,

src/xdisp.c

@@ -17323,6 +17323,7 @@ mark_window_display_accurate_1 (struct window *w, bool accurate_p)
 
       BUF_UNCHANGED_MODIFIED (b) = BUF_MODIFF (b);
       BUF_OVERLAY_UNCHANGED_MODIFIED (b) = BUF_OVERLAY_MODIFF (b);
+      BUF_CHARS_UNCHANGED_MODIFIED (b) = BUF_CHARS_MODIFF (b);
       BUF_BEG_UNCHANGED (b) = BUF_GPT (b) - BUF_BEG (b);
       BUF_END_UNCHANGED (b) = BUF_Z (b) - BUF_GPT (b);
 
@@ -19585,7 +19586,7 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
   /* Check whether the buffer to be displayed contains long lines.  */
   if (!NILP (Vlong_line_threshold)
       && !current_buffer->long_line_optimizations_p
-      && MODIFF - UNCHANGED_MODIFIED > 8)
+      && CHARS_MODIFF - CHARS_UNCHANGED_MODIFIED > 8)
     {
       ptrdiff_t cur, next, found, max = 0, threshold;
       threshold = XFIXNUM (Vlong_line_threshold);