vim-patch:9.1.2118: 'cursorline' missing after :diffput to empty buf (#37628)

Problem:  'cursorline' and part of 'statusline' are missing after
          :diffput to an empty buffer.
Solution: Make sure the cursor doesn't go beyond the last line after
          :diffput (zeertzjq)

related: neovim/neovim#37621
closes:  vim/vim#19281

ce1e562fda

src/nvim/diff.c

@@ -3890,6 +3890,10 @@ static void diffgetput(const int addr_count, const int idx_cur, const int idx_fr
           // lines.
           if (curwin->w_cursor.lnum >= lnum + count) {
             curwin->w_cursor.lnum += added;
+            // When the buffer was previously empty, the cursor may
+            // now be beyond the last line, so clamp cursor lnum.
+            curwin->w_cursor.lnum = MIN(curwin->w_cursor.lnum,
+                                        curbuf->b_ml.ml_line_count);
           } else if (added < 0) {
             curwin->w_cursor.lnum = lnum;
           }

test/functional/ui/diff_spec.lua

@@ -3379,3 +3379,50 @@ it(':%bwipe does not crash when using diffexpr', function()
     4 buffers wiped out                                                   |
   ]])
 end)
+
+-- oldtest: Test_diffput_to_empty_buf()
+it(':diffput to empty buffer redraws properly', function()
+  local screen = Screen.new(75, 20)
+  exec([[
+    set ruler
+    call setline(1, ['foo', 'bar', 'baz'])
+    rightbelow vnew
+    windo diffthis
+    windo set cursorline nofoldenable
+    wincmd t
+  ]])
+  screen:add_extra_attr_ids({
+    [100] = { underline = true, background = Screen.colors.LightBlue },
+  })
+  screen:expect([[
+    {7:  }{100:^foo                                }│{7:  }{23:-----------------------------------}|
+    {7:  }{22:bar                                }│{7:  }{23:-----------------------------------}|
+    {7:  }{22:baz                                }│{7:  }{23:-----------------------------------}|
+    {1:~                                    }│{7:  }{21:                                   }|
+    {1:~                                    }│{1:~                                    }|*14
+    {3:[No Name] [+]      1,1            All }{2:[No Name]          0,0-1          All}|
+                                                                               |
+  ]])
+  feed('0') -- Trigger an initial 'cursorbind' check.
+  screen:expect_unchanged()
+  command('diffput')
+  screen:expect([[
+    {7:  }{21:^foo                                }│{7:  }foo                                |
+    {7:  }bar                                │{7:  }bar                                |
+    {7:  }baz                                │{7:  }{21:baz                                }|
+    {1:~                                    }│{1:~                                    }|*15
+    {3:[No Name] [+]      1,1            All }{2:[No Name] [+]      3,1            All}|
+                                                                               |
+  ]])
+  command(':redraw!')
+  screen:expect_unchanged()
+  feed('j')
+  screen:expect([[
+    {7:  }foo                                │{7:  }foo                                |
+    {7:  }{21:^bar                                }│{7:  }{21:bar                                }|
+    {7:  }baz                                │{7:  }baz                                |
+    {1:~                                    }│{1:~                                    }|*15
+    {3:[No Name] [+]      2,1            All }{2:[No Name] [+]      2,1            All}|
+                                                                               |
+  ]])
+end)

test/old/testdir/test_diffmode.vim

@@ -3347,4 +3347,30 @@ func Test_diffexpr_wipe_buffers()
   call term_sendkeys(buf, ":so\<CR>")
   call WaitForAssert({-> assert_match('4 buffers wiped out', term_getline(buf, 20))})
 
+  call StopVimInTerminal(buf)
+endfunc
+
+func Test_diffput_to_empty_buf()
+  CheckScreendump
+
+  let lines =<< trim END
+    call setline(1, ['foo', 'bar', 'baz'])
+    rightbelow vnew
+    windo diffthis
+    windo set cursorline nofoldenable
+    wincmd t
+  END
+  call writefile(lines, 'Xtest_diffput_to_empty_buf', 'D')
+
+  let buf = RunVimInTerminal('-S Xtest_diffput_to_empty_buf', {})
+  call VerifyScreenDump(buf, 'Test_diffput_to_empty_buf_01', {})
+  call term_sendkeys(buf, '0')  " Trigger an initial 'cursorbind' check.
+  call VerifyScreenDump(buf, 'Test_diffput_to_empty_buf_01', {})
+  call term_sendkeys(buf, ":diffput | echo\<CR>")
+  call VerifyScreenDump(buf, 'Test_diffput_to_empty_buf_02', {})
+  call term_sendkeys(buf, ":redraw!\<CR>")
+  call VerifyScreenDump(buf, 'Test_diffput_to_empty_buf_02', {})
+  call term_sendkeys(buf, 'j')
+  call VerifyScreenDump(buf, 'Test_diffput_to_empty_buf_03', {})
+
   call StopVimInTerminal(buf)