vim-patch:8.2.4818: no test for what 8.2.4806 fixes

Problem:    No test for what 8.2.4806 fixes.
Solution:   Add a test. (closes vim/vim#10727)
ac92ab7719

Test cannot be used because it must use test_setmouse(). Use a Lua test.

src/nvim/testdir/test_mapping.vim

@@ -676,6 +676,35 @@ func Test_plug_remap()
   %bw!
 endfunc
 
+func Test_mouse_drag_mapped_start_select()
+  CheckFunction test_setmouse
+  set mouse=a
+  set selectmode=key,mouse
+  func ClickExpr()
+    call test_setmouse(1, 1)
+    return "\<LeftMouse>"
+  endfunc
+  func DragExpr()
+    call test_setmouse(1, 2)
+    return "\<LeftDrag>"
+  endfunc
+  nnoremap <expr> <F2> ClickExpr()
+  nmap <expr> <F3> DragExpr()
+
+  nnoremap <LeftDrag> <LeftDrag><Cmd><CR>
+  exe "normal \<F2>\<F3>"
+  call assert_equal('s', mode())
+  exe "normal! \<C-\>\<C-N>"
+
+  nunmap <LeftDrag>
+  nunmap <F2>
+  nunmap <F3>
+  delfunc ClickExpr
+  delfunc DragExpr
+  set selectmode&
+  set mouse&
+endfunc
+
 " Test for mapping <LeftDrag> in Insert mode
 func Test_mouse_drag_insert_map()
   CheckFunction test_setmouse

test/functional/legacy/mapping_spec.lua

@@ -129,10 +129,23 @@ describe('mapping', function()
       ]])
   end)
 
+  it('dragging starts Select mode even if coming from mapping vim-patch:8.2.4806', function()
+    command('set mouse=a')
+    command('set selectmode=mouse')
+
+    command('nnoremap <LeftDrag> <LeftDrag><Cmd><CR>')
+    sleep(10)
+    meths.input_mouse('left', 'press', '', 0, 0, 0)
+    sleep(10)
+    meths.input_mouse('left', 'drag', '', 0, 0, 1)
+    sleep(10)
+    eq('s', eval('mode()'))
+  end)
+
   it('<LeftDrag> mapping in Insert mode works correctly vim-patch:8.2.4692', function()
     command('set mouse=a')
 
-    command([[inoremap <LeftDrag> <LeftDrag><Cmd>let g:dragged = 1<CR>]])
+    command('inoremap <LeftDrag> <LeftDrag><Cmd>let g:dragged = 1<CR>')
     feed('i')
     sleep(10)
     meths.input_mouse('left', 'press', '', 0, 0, 0)