patch 9.0.1612: "skipcol" not reset when using multi-byte characters

Problem:    "skipcol" not reset when using multi-byte characters.
Solution:   Compare with w_virtcol instead of w_cursor.col. (closes #12457)

src/move.c

@@ -2414,10 +2414,14 @@ scroll_cursor_top(int min_scroll, int always)
 	}
 	check_topfill(curwin, FALSE);
 #endif
-	// TODO: if the line doesn't fit may optimize w_skipcol
-	if (curwin->w_topline == curwin->w_cursor.lnum
-		&& curwin->w_skipcol >= curwin->w_cursor.col)
-	    reset_skipcol();
+	if (curwin->w_topline == curwin->w_cursor.lnum)
+	{
+	    validate_virtcol();
+	    if (curwin->w_skipcol >= curwin->w_virtcol)
+		// TODO: if the line doesn't fit may optimize w_skipcol instead
+		// of making it zero
+		reset_skipcol();
+	}
 	if (curwin->w_topline != old_topline
 		|| curwin->w_skipcol != old_skipcol
 #ifdef FEAT_DIFF

src/testdir/dumps/Test_smoothscroll_multi_1.dump

@@ -0,0 +1,6 @@
+|ϛ+0&#ffffff0@34>ϛ@4
+@5| @34
+|2@35| @3
+|~+0#4040ff13&| @38
+|~| @38
+| +0#0000000&@21|1|,|7|1|-|3|6| @6|A|l@1| 

src/testdir/test_scroll_opt.vim

@@ -124,6 +124,21 @@ func Test_smoothscroll_CtrlE_CtrlY()
   call StopVimInTerminal(buf)
 endfunc
 
+func Test_smoothscroll_multibyte()
+  CheckScreendump
+
+  let lines =<< trim END
+      set scrolloff=0 smoothscroll
+      call setline(1, [repeat('ϛ', 45), repeat('2', 36)])
+      exe "normal G35l\<C-E>k"
+  END
+  call writefile(lines, 'XSmoothMultibyte', 'D')
+  let buf = RunVimInTerminal('-S XSmoothMultibyte', #{rows: 6, cols: 40})
+  call VerifyScreenDump(buf, 'Test_smoothscroll_multi_1', {})
+
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_smoothscroll_number()
   CheckScreendump
 

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1612,
 /**/
     1611,
 /**/