vim-patch:8.2.4555: getmousepos() returns the wrong column

Problem:    getmousepos() returns the wrong column. (Ernie Rael)
Solution:   Limit to the text size, not the number of bytes.
986b0fd0c5

test_setmouse is N/A; adjust test for Nvim.

N/A patches for version.c:

vim-patch:8.2.4569: Coverity warning for not using a return value

Problem:    Coverity warning for not using a return value.
Solution:   Add "(void)".
977525fea6

src/nvim/testdir/test_functions.vim

@@ -1762,6 +1762,36 @@ func Test_getcurpos_setpos()
   call assert_equal([0, 0, 0, 0, 0], getcurpos(1999))
 endfunc
 
+func Test_getmousepos()
+  enew!
+  call setline(1, "\t\t\t1234")
+  " call test_setmouse(1, 25)
+  call nvim_input_mouse('left', 'press', '', 0, 0, 24)
+  call getchar() " wait for and consume the mouse press
+  call assert_equal(#{
+        \ screenrow: 1,
+        \ screencol: 25,
+        \ winid: win_getid(),
+        \ winrow: 1,
+        \ wincol: 25,
+        \ line: 1,
+        \ column: 25,
+        \ }, getmousepos())
+  " call test_setmouse(1, 50)
+  call nvim_input_mouse('left', 'press', '', 0, 0, 49)
+  call getchar() " wait for and consume the mouse press
+  call assert_equal(#{
+        \ screenrow: 1,
+        \ screencol: 50,
+        \ winid: win_getid(),
+        \ winrow: 1,
+        \ wincol: 50,
+        \ line: 1,
+        \ column: 29,
+        \ }, getmousepos())
+  bwipe!
+endfunc
+
 func HasDefault(msg = 'msg')
   return a:msg
 endfunc