vim-patch:8.1.0767: when deleting lines at the bottom signs are misplaced

Problem:    When deleting lines at the bottom signs are misplaced.
Solution:   Properly update the line number of signs at the end of a buffer
            after a delete/undo operation. (Yegappan Lakshmanan, closes vim/vim#3798)
c771bf9016

src/nvim/sign.c

@@ -647,22 +647,30 @@ void sign_mark_adjust(linenr_T line1, linenr_T line2, long amount, long amount_a
     signlist_T *sign;    // a sign in a b_signlist
     signlist_T *next;    // the next sign in a b_signlist
     signlist_T **lastp;  // pointer to pointer to current sign
+    linenr_T new_lnum;   // new line number to assign to sign
 
     curbuf->b_signcols_max = -1;
     lastp = &curbuf->b_signlist;
 
     FOR_ALL_SIGNS_IN_BUF(curbuf, sign) {
         next = sign->next;
+	new_lnum = sign->lnum;
         if (sign->lnum >= line1 && sign->lnum <= line2) {
             if (amount == MAXLNUM) {
                 *lastp = next;
                 xfree(sign);
                 continue;
             } else {
-                sign->lnum += amount;
+                new_lnum += amount;
             }
         } else if (sign->lnum > line2) {
-            sign->lnum += amount_after;
+            new_lnum += amount_after;
+        }
+	// If the new sign line number is past the last line in the buffer,
+	// then don't adjust the line number. Otherwise, it will always be past
+	// the last line and will not be visible.
+	if (sign->lnum >= line1 && new_lnum <= curbuf->b_ml.ml_line_count) {
+	    sign->lnum = new_lnum;
 	}
         lastp = &sign->next;
     }

src/nvim/testdir/test_signs.vim

@@ -1202,13 +1202,13 @@ func Test_sign_lnum_adjust()
   enew! | only!
 
   sign define sign1 text=#> linehl=Comment
-  call setline(1, ['A', 'B', 'C', 'D'])
+  call setline(1, ['A', 'B', 'C', 'D', 'E'])
   exe 'sign place 5 line=3 name=sign1 buffer=' . bufnr('')
   let l = sign_getplaced(bufnr(''))
   call assert_equal(3, l[0].signs[0].lnum)
 
   " Add some lines before the sign and check the sign line number
-  call append(2, ['AA', 'AB', 'AC'])
+  call append(2, ['BA', 'BB', 'BC'])
   let l = sign_getplaced(bufnr(''))
   call assert_equal(6, l[0].signs[0].lnum)
 
@@ -1217,6 +1217,44 @@ func Test_sign_lnum_adjust()
   let l = sign_getplaced(bufnr(''))
   call assert_equal(4, l[0].signs[0].lnum)
 
+  " Insert some lines after the sign and check the sign line number
+  call append(5, ['DA', 'DB'])
+  let l = sign_getplaced(bufnr(''))
+  call assert_equal(4, l[0].signs[0].lnum)
+
+  " Delete some lines after the sign and check the sign line number
+  call deletebufline('', 6, 7)
+  let l = sign_getplaced(bufnr(''))
+  call assert_equal(4, l[0].signs[0].lnum)
+
+  " Break the undo. Otherwise the undo operation below will undo all the
+  " changes made by this function.
+  let &undolevels=&undolevels
+
+  " Delete the line with the sign
+  call deletebufline('', 4)
+  let l = sign_getplaced(bufnr(''))
+  call assert_equal(4, l[0].signs[0].lnum)
+
+  " Undo the delete operation
+  undo
+  let l = sign_getplaced(bufnr(''))
+  call assert_equal(5, l[0].signs[0].lnum)
+
+  " Break the undo
+  let &undolevels=&undolevels
+
+  " Delete few lines at the end of the buffer including the line with the sign
+  " Sign line number should not change (as it is placed outside of the buffer)
+  call deletebufline('', 3, 6)
+  let l = sign_getplaced(bufnr(''))
+  call assert_equal(5, l[0].signs[0].lnum)
+
+  " Undo the delete operation. Sign should be restored to the previous line
+  undo
+  let l = sign_getplaced(bufnr(''))
+  call assert_equal(5, l[0].signs[0].lnum)
+
   sign unplace * group=*
   sign undefine sign1
   enew!