1 files changed, 19 insertions, 6 deletions

diff --git a/src/nvim/fold.c b/src/nvim/fold.c
index 090f754e93..9c8e92cd00 100644
--- a/src/nvim/fold.c
+++ b/src/nvim/fold.c
@@ -775,7 +775,7 @@ void clearFolding(win_T *win)
 /// The changes in lines from top to bot (inclusive).
 void foldUpdate(win_T *wp, linenr_T top, linenr_T bot)
 {
-  if (compl_busy || State & MODE_INSERT) {
+  if (disable_fold_update || compl_busy || State & MODE_INSERT) {
     return;
   }
 
@@ -785,11 +785,18 @@ void foldUpdate(win_T *wp, linenr_T top, linenr_T bot)
   }
 
   if (wp->w_folds.ga_len > 0) {
-    // Mark all folds from top to bot as maybe-small.
+    linenr_T maybe_small_start = top;
+    linenr_T maybe_small_end = bot;
+
+    // Mark all folds from top to bot (or bot to top) as maybe-small.
+    if (top > bot) {
+      maybe_small_start = bot;
+      maybe_small_end = top;
+    }
     fold_T *fp;
-    (void)foldFind(&wp->w_folds, top, &fp);
+    (void)foldFind(&wp->w_folds, maybe_small_start, &fp);
     while (fp < (fold_T *)wp->w_folds.ga_data + wp->w_folds.ga_len
-           && fp->fd_top < bot) {
+           && fp->fd_top <= maybe_small_end) {
       fp->fd_small = kNone;
       fp++;
     }
@@ -1929,7 +1936,7 @@ static void foldUpdateIEMS(win_T *const wp, linenr_T top, linenr_T bot)
     bot = wp->w_buffer->b_ml.ml_line_count;
     wp->w_foldinvalid = false;
 
-    // Mark all folds a maybe-small.
+    // Mark all folds as maybe-small.
     setSmallMaybe(&wp->w_folds);
   }
 
@@ -1996,7 +2003,7 @@ static void foldUpdateIEMS(win_T *const wp, linenr_T top, linenr_T bot)
       // start one line back, because a "<1" may indicate the end of a
       // fold in the topline
       if (top > 1) {
-        --fline.lnum;
+        fline.lnum--;
       }
     } else if (foldmethodIsSyntax(wp)) {
       getlevel = foldlevelSyntax;
@@ -2004,6 +2011,12 @@ static void foldUpdateIEMS(win_T *const wp, linenr_T top, linenr_T bot)
       getlevel = foldlevelDiff;
     } else {
       getlevel = foldlevelIndent;
+      // Start one line back, because if the line above "top" has an
+      // undefined fold level, folding it relies on the line under it,
+      // which is "top".
+      if (top > 1) {
+        fline.lnum--;
+      }
     }
 
     // Backup to a line for which the fold level is defined.  Since it's