folds: Do not auto-update folds for some foldmethods. #5426

src/nvim/edit.c

@@ -462,8 +462,7 @@ static void insert_enter(InsertState *s)
     o_lnum = curwin->w_cursor.lnum;
   }
 
-  foldUpdateAll(curwin);
-  foldOpenCursor();
+  foldUpdateAfterInsert();
   if (s->cmdchar != 'r' && s->cmdchar != 'v') {
     apply_autocmds(EVENT_INSERTLEAVE, NULL, NULL, false, curbuf);
   }

src/nvim/fold.c

@@ -788,6 +788,19 @@ void foldUpdate(win_T *wp, linenr_T top, linenr_T bot)
   }
 }
 
+/// Updates folds when leaving insert-mode.
+void foldUpdateAfterInsert(void)
+{
+  if (foldmethodIsManual(curwin)  // foldmethod=manual: No need to update.
+      // These foldmethods are too slow, do not auto-update on insert-leave.
+      || foldmethodIsSyntax(curwin) || foldmethodIsExpr(curwin)) {
+    return;
+  }
+
+  foldUpdateAll(curwin);
+  foldOpenCursor();
+}
+
 /* foldUpdateAll() {{{2 */
 /*
  * Update all lines in a window for folding.

src/nvim/normal.c

@@ -5925,8 +5925,7 @@ static void nv_replace(cmdarg_T *cap)
     set_last_insert(cap->nchar);
   }
 
-  foldUpdateAll(curwin);
-  foldOpenCursor();
+  foldUpdateAfterInsert();
 }
 
 /*