patch 8.1.1216: mouse middle click is not tested

Problem:    Mouse middle click is not tested.
Solution:   Add a test. (Dominique Pelle, closes #4310)

src/testdir/test_termcodes.vim

@@ -26,10 +26,18 @@ func MouseLeftClick(row, col)
   call TerminalEscapeCode(0x20, 0, a:row, a:col, 'M')
 endfunc
 
+func MouseMiddleClick(row, col)
+  call TerminalEscapeCode(0x21, 1, a:row, a:col, 'M')
+endfunc
+
 func MouseLeftRelease(row, col)
   call TerminalEscapeCode(0x23, 3, a:row, a:col, 'm')
 endfunc
 
+func MouseMiddleRelease(row, col)
+  call TerminalEscapeCode(0x23, 3, a:row, a:col, 'm')
+endfunc
+
 func MouseLeftDrag(row, col)
   call TerminalEscapeCode(0x43, 0x20, a:row, a:col, 'M')
 endfunc
@@ -42,7 +50,7 @@ func MouseWheelDown(row, col)
   call TerminalEscapeCode(0x41, 0x41, a:row, a:col, 'M')
 endfunc
 
-func Test_xterm_mouse_click()
+func Test_xterm_mouse_left_click()
   new
   let save_mouse = &mouse
   let save_term = &term
@@ -68,6 +76,48 @@ func Test_xterm_mouse_click()
   bwipe!
 endfunc
 
+func Test_xterm_mouse_middle_click()
+  new
+  let save_mouse = &mouse
+  let save_term = &term
+  let save_ttymouse = &ttymouse
+  let save_quotestar = @*
+  let @* = 'abc'
+  set mouse=a term=xterm
+
+  for ttymouse_val in ['xterm2', 'sgr']
+    let msg = 'ttymouse=' .. ttymouse_val
+    exe 'set ttymouse=' . ttymouse_val
+    call setline(1, ['123456789', '123456789'])
+
+    " Middle-click in the middle of the line pastes text where clicked.
+    let row = 1
+    let col = 6
+    call MouseMiddleClick(row, col)
+    call MouseMiddleRelease(row, col)
+    call assert_equal(['12345abc6789', '123456789'], getline(1, '$'), msg)
+
+    " Middle-click beyond end of the line pastes text at the end of the line.
+    let col = 20
+    call MouseMiddleClick(row, col)
+    call MouseMiddleRelease(row, col)
+    call assert_equal(['12345abc6789abc', '123456789'], getline(1, '$'), msg)
+
+    " Middle-click beyond the last line pastes in the last line.
+    let row = 5
+    let col = 3
+    call MouseMiddleClick(row, col)
+    call MouseMiddleRelease(row, col)
+    call assert_equal(['12345abc6789abc', '12abc3456789'], getline(1, '$'), msg)
+  endfor
+
+  let &mouse = save_mouse
+  let &term = save_term
+  let &ttymouse = save_ttymouse
+  let @* = save_quotestar
+  bwipe!
+endfunc
+
 func Test_xterm_mouse_wheel()
   new
   let save_mouse = &mouse

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1216,
 /**/
     1215,
 /**/