patch 8.2.4813: pasting text while indent folding may mess up folds

Problem:    Pasting text while indent folding may mess up folds.
Solution:   Adjust the way folds are split. (Brandon Simmons, closes #10254)

src/fold.c

@@ -1128,7 +1128,7 @@ cloneFoldGrowArray(garray_T *from, garray_T *to)
 // foldFind() {{{2
 /*
  * Search for line "lnum" in folds of growarray "gap".
- * Set *fpp to the fold struct for the fold that contains "lnum" or
+ * Set "*fpp" to the fold struct for the fold that contains "lnum" or
  * the first fold below it (careful: it can be beyond the end of the array!).
  * Returns FALSE when there is no fold that contains "lnum".
  */
@@ -2911,7 +2911,8 @@ foldSplit(
     // any between top and bot, they have been removed by the caller.
     gap1 = &fp->fd_nested;
     gap2 = &fp[1].fd_nested;
-    if (foldFind(gap1, bot + 1 - fp->fd_top, &fp2))
+    (void)foldFind(gap1, bot + 1 - fp->fd_top, &fp2);
+    if (fp2 != NULL)
     {
 	len = (int)((fold_T *)gap1->ga_data + gap1->ga_len - fp2);
 	if (len > 0 && ga_grow(gap2, len) == OK)

src/testdir/test_fold.vim

@@ -1439,4 +1439,24 @@ func Test_foldtext_scriptlocal_func()
   delfunc s:FoldText
 endfunc
 
+" Make sure a fold containing a nested fold is split correctly when using
+" foldmethod=indent
+func Test_fold_split()
+  new
+  let lines =<< trim END
+    line 1
+      line 2
+      line 3
+        line 4
+        line 5
+  END
+  call setline(1, lines)
+  setlocal sw=2
+  setlocal foldmethod=indent foldenable
+  call assert_equal([0, 1, 1, 2, 2], range(1, 5)->map('foldlevel(v:val)'))
+  call append(2, 'line 2.5')
+  call assert_equal([0, 1, 0, 1, 2, 2], range(1, 6)->map('foldlevel(v:val)'))
+  bw!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4813,
 /**/
     4812,
 /**/