vim-patch:9.1.0743: diff mode does not handle overlapping diffs correctly (#30532)

Problem:  diff mode does not handle overlapping diffs correctly
Solution: correct the logic to handle overlapping blocks
          (Yukihiro Nakadaira)

Vim merges overlapped diff blocks and it doesn't work expectedly
in some situation.

closes: vim/vim#15735

https://github.com/vim/vim/commit/06fe70c183a53ea97cd42ace490d4fb9fd14f042

Co-authored-by: Yukihiro Nakadaira <yukihiro.nakadaira@gmail.com>

3 files changed, 751 insertions, 10 deletions

diff --git a/src/nvim/diff.c b/src/nvim/diff.c
index 0c59b8eb67..05e5bed50c 100644
--- a/src/nvim/diff.c
+++ b/src/nvim/diff.c
@@ -1609,6 +1609,7 @@ static void process_hunk(diff_T **dpp, diff_T **dprevp, int idx_orig, int idx_ne
       for (int i = idx_orig; i < idx_new; i++) {
         if (curtab->tp_diffbuf[i] != NULL) {
           dp->df_lnum[i] -= off;
+          dp->df_count[i] += off;
         }
       }
       dp->df_lnum[idx_new] = hunk->lnum_new;
@@ -1619,11 +1620,7 @@ static void process_hunk(diff_T **dpp, diff_T **dprevp, int idx_orig, int idx_ne
       dp->df_count[idx_new] = (linenr_T)hunk->count_new - off;
     } else {
       // second overlap of new block with existing block
-      dp->df_count[idx_new] += (linenr_T)hunk->count_new - (linenr_T)hunk->count_orig
-                               + dpl->df_lnum[idx_orig] +
-                               dpl->df_count[idx_orig]
-                               - (dp->df_lnum[idx_orig] +
-                                  dp->df_count[idx_orig]);
+      dp->df_count[idx_new] += (linenr_T)hunk->count_new;
     }
 
     // Adjust the size of the block to include all the lines to the
@@ -1632,11 +1629,8 @@ static void process_hunk(diff_T **dpp, diff_T **dprevp, int idx_orig, int idx_ne
           - (dpl->df_lnum[idx_orig] + dpl->df_count[idx_orig]);
 
     if (off < 0) {
-      // new change ends in existing block, adjust the end if not
-      // done already
-      if (*notsetp) {
-        dp->df_count[idx_new] += -off;
-      }
+      // new change ends in existing block, adjust the end
+      dp->df_count[idx_new] += -off;
       off = 0;
     }
 
diff --git a/test/functional/ui/diff_spec.lua b/test/functional/ui/diff_spec.lua
index 8db6f776d1..d6a04f90f6 100644
--- a/test/functional/ui/diff_spec.lua
+++ b/test/functional/ui/diff_spec.lua
@@ -12,6 +12,19 @@ local exec = n.exec
 local eq = t.eq
 local api = n.api
 
+local function WriteDiffFiles(text1, text2)
+  write_file('Xdifile1', text1)
+  write_file('Xdifile2', text2)
+  command('checktime')
+end
+
+local function WriteDiffFiles3(text1, text2, text3)
+  write_file('Xdifile1', text1)
+  write_file('Xdifile2', text2)
+  write_file('Xdifile3', text3)
+  command('checktime')
+end
+
 before_each(clear)
 
 describe('Diff mode screen', function()
@@ -1515,3 +1528,529 @@ it("diff mode draws 'breakindent' correctly after filler lines", function()
                                                  |
   ]])
 end)
+
+-- oldtest: Test_diff_overlapped_diff_blocks_will_be_merged()
+it('diff mode overlapped diff blocks will be merged', function()
+  write_file('Xdifile1', '')
+  write_file('Xdifile2', '')
+  write_file('Xdifile3', '')
+
+  finally(function()
+    os.remove('Xdifile1')
+    os.remove('Xdifile2')
+    os.remove('Xdifile3')
+    os.remove('Xdiin1')
+    os.remove('Xdinew1')
+    os.remove('Xdiout1')
+    os.remove('Xdiin2')
+    os.remove('Xdinew2')
+    os.remove('Xdiout2')
+  end)
+
+  exec([[
+    func DiffExprStub()
+      let txt_in = readfile(v:fname_in)
+      let txt_new = readfile(v:fname_new)
+      if txt_in == ["line1"] && txt_new == ["line2"]
+        call writefile(["1c1"], v:fname_out)
+      elseif txt_in == readfile("Xdiin1") && txt_new == readfile("Xdinew1")
+        call writefile(readfile("Xdiout1"), v:fname_out)
+      elseif txt_in == readfile("Xdiin2") && txt_new == readfile("Xdinew2")
+        call writefile(readfile("Xdiout2"), v:fname_out)
+      endif
+    endfunc
+  ]])
+
+  local screen = Screen.new(35, 20)
+  screen:attach()
+  command('set winwidth=10 diffopt=filler,internal')
+
+  command('args Xdifile1 Xdifile2 | vert all | windo diffthis')
+
+  WriteDiffFiles('a\nb', 'x\nx')
+  write_file('Xdiin1', 'a\nb')
+  write_file('Xdinew1', 'x\nx')
+  write_file('Xdiout1', '1c1\n2c2')
+  command('set diffexpr=DiffExprStub()')
+  screen:expect([[
+    {7:  }{27:a}{4:              }│{7:  }{27:^x}{4:              }|
+    {7:  }{27:b}{4:              }│{7:  }{27:x}{4:              }|
+    {1:~                }│{1:~                }|*16
+    {2:Xdifile1          }{3:Xdifile2         }|
+                                       |
+  ]])
+  command('set diffexpr&')
+
+  WriteDiffFiles('a\nb\nc', 'x\nc')
+  write_file('Xdiin1', 'a\nb\nc')
+  write_file('Xdinew1', 'x\nc')
+  write_file('Xdiout1', '1c1\n2c1')
+  command('set diffexpr=DiffExprStub()')
+  screen:expect([[
+    {7:  }{27:a}{4:              }│{7:  }{27:^x}{4:              }|
+    {7:  }{22:b              }│{7:  }{23:---------------}|
+    {7:  }c              │{7:  }c              |
+    {1:~                }│{1:~                }|*15
+    {2:Xdifile1          }{3:Xdifile2         }|
+                                       |
+  ]])
+  command('set diffexpr&')
+
+  WriteDiffFiles('a\nc', 'x\nx\nc')
+  write_file('Xdiin1', 'a\nc')
+  write_file('Xdinew1', 'x\nx\nc')
+  write_file('Xdiout1', '1c1\n1a2')
+  command('set diffexpr=DiffExprStub()')
+  screen:expect([[
+    {7:  }{27:a}{4:              }│{7:  }{27:^x}{4:              }|
+    {7:  }{23:---------------}│{7:  }{22:x              }|
+    {7:  }c              │{7:  }c              |
+    {1:~                }│{1:~                }|*15
+    {2:Xdifile1          }{3:Xdifile2         }|
+                                       |
+  ]])
+  command('set diffexpr&')
+
+  command('args Xdifile1 Xdifile2 Xdifile3 | vert all | windo diffthis')
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'y\nb\nc')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:a}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:b}{4:        }|
+    {7:  }c        │{7:  }c        │{7:  }c        |
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'a\ny\nc')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }c        │{7:  }c        │{7:  }c        |
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'a\nb\ny')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:b}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'y\ny\nc')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:a}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }c        │{7:  }c        │{7:  }c        |
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'a\ny\ny')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'y\ny\ny')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:a}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nx', 'y\ny\nc')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:a}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:c}{4:        }|
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'x\nx\nc', 'a\ny\ny')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:^a}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'y\ny\ny\nd\ne')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:a}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:d}{4:        }|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'y\ny\ny\ny\ne')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:a}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'y\ny\ny\ny\ny')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:a}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:e}{4:        }│{7:  }{27:e}{4:        }│{7:  }{27:y}{4:        }|
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\ny\ny\nd\ne')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:d}{4:        }|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\ny\ny\ny\ne')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\ny\ny\ny\ny')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:e}{4:        }│{7:  }{27:e}{4:        }│{7:  }{27:y}{4:        }|
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\nb\ny\nd\ne')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:b}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:d}{4:        }|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\nb\ny\ny\ne')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:b}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\nb\ny\ny\ny')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:b}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:e}{4:        }│{7:  }{27:e}{4:        }│{7:  }{27:y}{4:        }|
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb', 'x\nb', 'y\ny')
+  write_file('Xdiin1', 'a\nb')
+  write_file('Xdinew1', 'x\nb')
+  write_file('Xdiout1', '1c1')
+  write_file('Xdiin2', 'a\nb')
+  write_file('Xdinew2', 'y\ny')
+  write_file('Xdiout2', '1c1\n2c2')
+  command('set diffexpr=DiffExprStub()')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:b}{4:        }│{7:  }{27:y}{4:        }|
+    {1:~          }│{1:~          }│{1:~          }|*16
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+  command('set diffexpr&')
+
+  WriteDiffFiles3('a\nb\nc\nd', 'x\nb\nx\nd', 'y\ny\nc\nd')
+  write_file('Xdiin1', 'a\nb\nc\nd')
+  write_file('Xdinew1', 'x\nb\nx\nd')
+  write_file('Xdiout1', '1c1\n3c3')
+  write_file('Xdiin2', 'a\nb\nc\nd')
+  write_file('Xdinew2', 'y\ny\nc\nd')
+  write_file('Xdiout2', '1c1\n2c2')
+  command('set diffexpr=DiffExprStub()')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:b}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:c}{4:        }|
+    {7:  }d        │{7:  }d        │{7:  }d        |
+    {1:~          }│{1:~          }│{1:~          }|*14
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+  command('set diffexpr&')
+
+  WriteDiffFiles3('a\nb\nc\nd', 'x\nb\nx\nd', 'y\ny\ny\nd')
+  write_file('Xdiin1', 'a\nb\nc\nd')
+  write_file('Xdinew1', 'x\nb\nx\nd')
+  write_file('Xdiout1', '1c1\n3c3')
+  write_file('Xdiin2', 'a\nb\nc\nd')
+  write_file('Xdinew2', 'y\ny\ny\nd')
+  write_file('Xdiout2', '1c1\n2,3c2,3')
+  command('set diffexpr=DiffExprStub()')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:b}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }d        │{7:  }d        │{7:  }d        |
+    {1:~          }│{1:~          }│{1:~          }|*14
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+  command('set diffexpr&')
+
+  WriteDiffFiles3('a\nb\nc\nd', 'x\nb\nx\nd', 'y\ny\ny\ny')
+  write_file('Xdiin1', 'a\nb\nc\nd')
+  write_file('Xdinew1', 'x\nb\nx\nd')
+  write_file('Xdiout1', '1c1\n3c3')
+  write_file('Xdiin2', 'a\nb\nc\nd')
+  write_file('Xdinew2', 'y\ny\ny\ny')
+  write_file('Xdiout2', '1c1\n2,4c2,4')
+  command('set diffexpr=DiffExprStub()')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:^y}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:b}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:d}{4:        }│{7:  }{27:y}{4:        }|
+    {1:~          }│{1:~          }│{1:~          }|*14
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+  command('set diffexpr&')
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'b\nc')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:a}{4:        }│{7:  }{27:^b}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }c        │{7:  }c        │{7:  }c        |
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'c')
+  screen:expect([[
+    {7:  }{4:a        }│{7:  }{4:a        }│{7:  }{23:---------}|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }c        │{7:  }c        │{7:  }^c        |
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', '')
+  screen:expect([[
+    {7:  }{4:a        }│{7:  }{4:a        }│{7:  }{23:---------}|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }{4:c        }│{7:  }{4:c        }│{7:  }{23:---------}|
+    {1:~          }│{1:~          }│{7:  }^         |
+    {1:~          }│{1:~          }│{1:~          }|*14
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'a\nc')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }c        │{7:  }c        │{7:  }c        |
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'a')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }{4:c        }│{7:  }{4:c        }│{7:  }{23:---------}|
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'b')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:a}{4:        }│{7:  }{27:^b}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }{4:c        }│{7:  }{4:c        }│{7:  }{23:---------}|
+    {1:~          }│{1:~          }│{1:~          }|*15
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'd\ne')
+  screen:expect([[
+    {7:  }{27:a}{4:        }│{7:  }{27:a}{4:        }│{7:  }{27:^d}{4:        }|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }{4:c        }│{7:  }{4:c        }│{7:  }{23:---------}|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'e')
+  screen:expect([[
+    {7:  }{4:a        }│{7:  }{4:a        }│{7:  }{23:---------}|
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }{4:c        }│{7:  }{4:c        }│{7:  }{23:---------}|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }e        │{7:  }e        │{7:  }^e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\nd\ne')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:d}{4:        }|
+    {7:  }{4:c        }│{7:  }{4:c        }│{7:  }{23:---------}|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\ne')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }{4:c        }│{7:  }{4:c        }│{7:  }{23:---------}|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }{4:c        }│{7:  }{4:c        }│{7:  }{23:---------}|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }{4:e        }│{7:  }{4:e        }│{7:  }{23:---------}|
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\nb\nd\ne')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:b}{4:        }|
+    {7:  }{27:c}{4:        }│{7:  }{27:c}{4:        }│{7:  }{27:d}{4:        }|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\nb\ne')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:b}{4:        }|
+    {7:  }{4:c        }│{7:  }{4:c        }│{7:  }{23:---------}|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }e        │{7:  }e        │{7:  }e        |
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc\nd\ne', 'a\nx\nc\nx\ne', 'a\nb')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:b}{4:        }|
+    {7:  }{4:c        }│{7:  }{4:c        }│{7:  }{23:---------}|
+    {7:  }{27:d}{4:        }│{7:  }{27:x}{4:        }│{7:  }{23:---------}|
+    {7:  }{4:e        }│{7:  }{4:e        }│{7:  }{23:---------}|
+    {1:~          }│{1:~          }│{1:~          }|*13
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'a\ny\nb\nc')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:y}{4:        }|
+    {7:  }{23:---------}│{7:  }{23:---------}│{7:  }{22:b        }|
+    {7:  }c        │{7:  }c        │{7:  }c        |
+    {1:~          }│{1:~          }│{1:~          }|*14
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+
+  WriteDiffFiles3('a\nb\nc', 'a\nx\nc', 'a\nb\ny\nc')
+  screen:expect([[
+    {7:  }a        │{7:  }a        │{7:  }^a        |
+    {7:  }{27:b}{4:        }│{7:  }{27:x}{4:        }│{7:  }{27:b}{4:        }|
+    {7:  }{23:---------}│{7:  }{23:---------}│{7:  }{22:y        }|
+    {7:  }c        │{7:  }c        │{7:  }c        |
+    {1:~          }│{1:~          }│{1:~          }|*14
+    {2:Xdifile1    Xdifile2    }{3:Xdifile3   }|
+                                       |
+  ]])
+end)
diff --git a/test/old/testdir/test_diffmode.vim b/test/old/testdir/test_diffmode.vim
index 84cb7eb376..a448ed9b7f 100644
--- a/test/old/testdir/test_diffmode.vim
+++ b/test/old/testdir/test_diffmode.vim
@@ -830,6 +830,15 @@ func WriteDiffFiles(buf, list1, list2)
   endif
 endfunc
 
+func WriteDiffFiles3(buf, list1, list2, list3)
+  call writefile(a:list1, 'Xdifile1')
+  call writefile(a:list2, 'Xdifile2')
+  call writefile(a:list3, 'Xdifile3')
+  if a:buf
+    call term_sendkeys(a:buf, ":checktime\<CR>")
+  endif
+endfunc
+
 " Verify a screendump with both the internal and external diff.
 func VerifyBoth(buf, dumpfile, extra)
   " trailing : for leaving the cursor on the command line
@@ -1799,4 +1808,203 @@ func Test_diff_eob_halfpage()
   %bwipe!
 endfunc
 
+func Test_diff_overlapped_diff_blocks_will_be_merged()
+  CheckScreendump
+
+  let lines =<< trim END
+    func DiffExprStub()
+      let txt_in = readfile(v:fname_in)
+      let txt_new = readfile(v:fname_new)
+      if txt_in == ["line1"] && txt_new == ["line2"]
+        call writefile(["1c1"], v:fname_out)
+      elseif txt_in == readfile("Xdiin1") && txt_new == readfile("Xdinew1")
+        call writefile(readfile("Xdiout1"), v:fname_out)
+      elseif txt_in == readfile("Xdiin2") && txt_new == readfile("Xdinew2")
+        call writefile(readfile("Xdiout2"), v:fname_out)
+      endif
+    endfunc
+  END
+  call writefile(lines, 'XdiffSetup', 'D')
+
+  call WriteDiffFiles(0, [], [])
+  let buf = RunVimInTerminal('-d -S XdiffSetup Xdifile1 Xdifile2', {})
+  call term_sendkeys(buf, ":set autoread\<CR>\<c-w>w:set autoread\<CR>\<c-w>w")
+
+  call WriteDiffFiles(buf, ["a", "b"], ["x", "x"])
+  call writefile(["a", "b"], "Xdiin1")
+  call writefile(["x", "x"], "Xdinew1")
+  call writefile(["1c1", "2c2"], "Xdiout1")
+  call term_sendkeys(buf, ":set diffexpr=DiffExprStub()\<CR>:")
+  call VerifyBoth(buf, "Test_diff_overlapped_2.01", "")
+  call term_sendkeys(buf, ":set diffexpr&\<CR>:")
+
+  call WriteDiffFiles(buf, ["a", "b", "c"], ["x", "c"])
+  call writefile(["a", "b", "c"], "Xdiin1")
+  call writefile(["x", "c"], "Xdinew1")
+  call writefile(["1c1", "2d1"], "Xdiout1")
+  call term_sendkeys(buf, ":set diffexpr=DiffExprStub()\<CR>:")
+  call VerifyBoth(buf, "Test_diff_overlapped_2.02", "")
+  call term_sendkeys(buf, ":set diffexpr&\<CR>:")
+
+  call WriteDiffFiles(buf, ["a", "c"], ["x", "x", "c"])
+  call writefile(["a", "c"], "Xdiin1")
+  call writefile(["x", "x", "c"], "Xdinew1")
+  call writefile(["1c1", "1a2"], "Xdiout1")
+  call term_sendkeys(buf, ":set diffexpr=DiffExprStub()\<CR>:")
+  call VerifyBoth(buf, "Test_diff_overlapped_2.03", "")
+  call term_sendkeys(buf, ":set diffexpr&\<CR>:")
+
+  call StopVimInTerminal(buf)
+  wincmd c
+
+  call WriteDiffFiles3(0, [], [], [])
+  let buf = RunVimInTerminal('-d -S XdiffSetup Xdifile1 Xdifile2 Xdifile3', {})
+  call term_sendkeys(buf, ":set autoread\<CR>\<c-w>w:set autoread\<CR>\<c-w>w:set autoread\<CR>\<c-w>w")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["y", "b", "c"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.01", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["a", "y", "c"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.02", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["a", "b", "y"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.03", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["y", "y", "c"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.04", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["a", "y", "y"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.05", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["y", "y", "y"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.06", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "x"], ["y", "y", "c"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.07", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["x", "x", "c"], ["a", "y", "y"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.08", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["y", "y", "y", "d", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.09", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["y", "y", "y", "y", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.10", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["y", "y", "y", "y", "y"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.11", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "y", "y", "d", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.12", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "y", "y", "y", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.13", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "y", "y", "y", "y"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.14", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "b", "y", "d", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.15", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "b", "y", "y", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.16", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "b", "y", "y", "y"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.17", "")
+
+  call WriteDiffFiles3(buf, ["a", "b"], ["x", "b"], ["y", "y"])
+  call writefile(["a", "b"], "Xdiin1")
+  call writefile(["x", "b"], "Xdinew1")
+  call writefile(["1c1"], "Xdiout1")
+  call writefile(["a", "b"], "Xdiin2")
+  call writefile(["y", "y"], "Xdinew2")
+  call writefile(["1c1", "2c2"], "Xdiout2")
+  call term_sendkeys(buf, ":set diffexpr=DiffExprStub()\<CR>:")
+  call VerifyInternal(buf, "Test_diff_overlapped_3.18", "")
+  call term_sendkeys(buf, ":set diffexpr&\<CR>:")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d"], ["x", "b", "x", "d"], ["y", "y", "c", "d"])
+  call writefile(["a", "b", "c", "d"], "Xdiin1")
+  call writefile(["x", "b", "x", "d"], "Xdinew1")
+  call writefile(["1c1", "3c3"], "Xdiout1")
+  call writefile(["a", "b", "c", "d"], "Xdiin2")
+  call writefile(["y", "y", "c", "d"], "Xdinew2")
+  call writefile(["1c1", "2c2"], "Xdiout2")
+  call term_sendkeys(buf, ":set diffexpr=DiffExprStub()\<CR>:")
+  call VerifyInternal(buf, "Test_diff_overlapped_3.19", "")
+  call term_sendkeys(buf, ":set diffexpr&\<CR>:")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d"], ["x", "b", "x", "d"], ["y", "y", "y", "d"])
+  call writefile(["a", "b", "c", "d"], "Xdiin1")
+  call writefile(["x", "b", "x", "d"], "Xdinew1")
+  call writefile(["1c1", "3c3"], "Xdiout1")
+  call writefile(["a", "b", "c", "d"], "Xdiin2")
+  call writefile(["y", "y", "y", "d"], "Xdinew2")
+  call writefile(["1c1", "2,3c2,3"], "Xdiout2")
+  call term_sendkeys(buf, ":set diffexpr=DiffExprStub()\<CR>:")
+  call VerifyInternal(buf, "Test_diff_overlapped_3.20", "")
+  call term_sendkeys(buf, ":set diffexpr&\<CR>:")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d"], ["x", "b", "x", "d"], ["y", "y", "y", "y"])
+  call writefile(["a", "b", "c", "d"], "Xdiin1")
+  call writefile(["x", "b", "x", "d"], "Xdinew1")
+  call writefile(["1c1", "3c3"], "Xdiout1")
+  call writefile(["a", "b", "c", "d"], "Xdiin2")
+  call writefile(["y", "y", "y", "y"], "Xdinew2")
+  call writefile(["1c1", "2,4c2,4"], "Xdiout2")
+  call term_sendkeys(buf, ":set diffexpr=DiffExprStub()\<CR>:")
+  call VerifyInternal(buf, "Test_diff_overlapped_3.21", "")
+  call term_sendkeys(buf, ":set diffexpr&\<CR>:")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["b", "c"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.22", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["c"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.23", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], [])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.24", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["a", "c"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.25", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["a"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.26", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["b"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.27", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["d", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.28", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.29", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "d", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.30", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.31", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.32", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "b", "d", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.33", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "b", "e"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.34", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c", "d", "e"], ["a", "x", "c", "x", "e"], ["a", "b"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.35", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["a", "y", "b", "c"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.36", "")
+
+  call WriteDiffFiles3(buf, ["a", "b", "c"], ["a", "x", "c"], ["a", "b", "y", "c"])
+  call VerifyBoth(buf, "Test_diff_overlapped_3.37", "")
+
+  call StopVimInTerminal(buf)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab