patch 9.0.1742: wrong curswant when clicking on second cell of double-width char

Problem:  Wrong curswant when clicking and the second cell of a
          double-width char.
Solution: Don't copy virtcol of the first char to the second one.

closes: #12842

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/screen.c

@@ -17,7 +17,7 @@
  * ScreenLines[off]  Contains a copy of the whole screen, as it is currently
  *		     displayed (excluding text written by external commands).
  * ScreenAttrs[off]  Contains the associated attributes.
- * ScreenCols[off]   Contains the byte offset in the line. -1 means not
+ * ScreenCols[off]   Contains the virtual columns in the line. -1 means not
  *		     available (below last line), MAXCOL means after the end
  *		     of the line.
  *
@@ -743,7 +743,7 @@ screen_line(
 
 	ScreenCols[off_to] = ScreenCols[off_from];
 	if (char_cells == 2)
-	    ScreenCols[off_to + 1] = ScreenCols[off_from];
+	    ScreenCols[off_to + 1] = ScreenCols[off_from + 1];
 
 	off_to += char_cells;
 	off_from += char_cells;

src/testdir/test_normal.vim

@@ -4079,4 +4079,29 @@ func Test_normal_click_on_ctrl_char()
   let &mouse = save_mouse
 endfunc
 
+" Test clicking on a double-width character in Normal mode
+func Test_normal_click_on_double_width_char()
+  let save_mouse = &mouse
+  set mouse=a
+  new
+
+  call setline(1, "口口")
+  redraw
+  call test_setmouse(1, 1)
+  call feedkeys("\<LeftMouse>", 'xt')
+  call assert_equal([0, 1, 1, 0, 1], getcurpos())
+  call test_setmouse(1, 2)
+  call feedkeys("\<LeftMouse>", 'xt')
+  call assert_equal([0, 1, 1, 0, 2], getcurpos())
+  call test_setmouse(1, 3)
+  call feedkeys("\<LeftMouse>", 'xt')
+  call assert_equal([0, 1, 4, 0, 3], getcurpos())
+  call test_setmouse(1, 4)
+  call feedkeys("\<LeftMouse>", 'xt')
+  call assert_equal([0, 1, 4, 0, 4], getcurpos())
+
+  bwipe!
+  let &mouse = save_mouse
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1742,
 /**/
     1741,
 /**/