patch 8.2.4568: getmousepos() does not compute the column below the last line

Problem:    getmousepos() does not compute the column below the last line.
Solution:   Also compute the column when the mouse is below the last line.
            (Sean Dewar, closes #9946)

src/testdir/test_functions.vim

@@ -2783,6 +2783,29 @@ func Test_getmousepos()
         \ line: 1,
         \ column: 8,
         \ }, getmousepos())
+
+  " If the mouse is positioned past the last buffer line, "line" and "column"
+  " should act like it's positioned on the last buffer line.
+  call test_setmouse(2, 25)
+  call assert_equal(#{
+        \ screenrow: 2,
+        \ screencol: 25,
+        \ winid: win_getid(),
+        \ winrow: 2,
+        \ wincol: 25,
+        \ line: 1,
+        \ column: 4,
+        \ }, getmousepos())
+  call test_setmouse(2, 50)
+  call assert_equal(#{
+        \ screenrow: 2,
+        \ screencol: 50,
+        \ winid: win_getid(),
+        \ winrow: 2,
+        \ wincol: 50,
+        \ line: 1,
+        \ column: 8,
+        \ }, getmousepos())
   bwipe!
 endfunc