fix(column): keep track of number of lines with number of signs

Problem:  Some edge cases to the old (pre-#26406) and current "b_signcols"
          structure result in an incorrectly sized "auto" 'signcolumn'.
Solution: * Implement a simpler 'signcolumn' validation strategy by immediately
            counting the number of signs in a range upon sign insertion and
            deletion. Decrease in performance here but there is a clear path
            forward to decreasing this performance hit by moving signs to a
            dedicated marktree, or by adding meta-data to the existing
            marktree which may be queried more efficiently?
          * Also replace "max_count" and keep track of the number of lines with
            a certain number of signs. This makes it so that it is no longer
            necessary to scan the entire buffer when the maximum number of signs
            decreases. This likely makes the commit a net increase in performance.
          * To ensure correctness we also have re-initialize the count for an
            edited region that spans multiple lines. Such an edit may move the
            signs within it. Thus we count and decrement before splicing the
            marktree and count and increment after.

1 files changed, 19 insertions, 16 deletions

diff --git a/src/nvim/drawscreen.c b/src/nvim/drawscreen.c
index 7f794a58d2..b49de19349 100644
--- a/src/nvim/drawscreen.c
+++ b/src/nvim/drawscreen.c
@@ -1201,27 +1201,30 @@ void comp_col(void)
 /// Redraw entire window "wp" if configured 'signcolumn' width changes.
 static bool win_redraw_signcols(win_T *wp)
 {
-  int width;
-  bool rebuild_stc = false;
   buf_T *buf = wp->w_buffer;
 
-  if (wp->w_minscwidth <= SCL_NO) {
-    if (*wp->w_p_stc) {
-      buf_signcols_validate(wp, buf, true);
-      if (buf->b_signcols.resized) {
-        rebuild_stc = true;
-        wp->w_nrwidth_line_count = 0;
-      }
-    }
-    width = 0;
-  } else if (wp->w_maxscwidth <= 1 && buf->b_signs_with_text >= (size_t)wp->w_maxscwidth) {
-    width = wp->w_maxscwidth;
-  } else {
-    width = MIN(wp->w_maxscwidth, buf_signcols_validate(wp, buf, false));
+  if (!buf->b_signcols.autom
+      && (*wp->w_p_stc != NUL || (wp->w_maxscwidth > 1 && wp->w_minscwidth != wp->w_maxscwidth))) {
+    buf->b_signcols.autom = true;
+    buf_signcols_count_range(buf, 0, buf->b_ml.ml_line_count, MAXLNUM, kNone);
+  }
+
+  while (buf->b_signcols.max > 0 && buf->b_signcols.count[buf->b_signcols.max - 1] == 0) {
+    buf->b_signcols.resized = true;
+    buf->b_signcols.max--;
+  }
+
+  int width = MIN(wp->w_maxscwidth, buf->b_signcols.max);
+  bool rebuild_stc = buf->b_signcols.resized && *wp->w_p_stc != NUL;
+
+  if (rebuild_stc) {
+    wp->w_nrwidth_line_count = 0;
+  } else if (wp->w_minscwidth == 0 && wp->w_maxscwidth == 1) {
+    width = buf->b_signs_with_text > 0;
   }
 
   int scwidth = wp->w_scwidth;
-  wp->w_scwidth = MAX(wp->w_minscwidth, width);
+  wp->w_scwidth = MAX(MAX(0, wp->w_minscwidth), width);
   return (wp->w_scwidth != scwidth || rebuild_stc);
 }