patch 9.0.1602: stray character visible if marker on top of double-wide char

Problem:    Stray character is visible if 'smoothscroll' marker is displayed
            on top of a double-wide character.
Solution:   When overwriting a double-width character with the 'smoothscroll'
            marker clear the second half. (closes #12469)

src/drawline.c

@@ -823,6 +823,7 @@ wlv_screen_line(win_T *wp, winlinevars_T *wlv, int negative_width)
 	    && !(wp->w_p_list && wp->w_lcs_chars.prec != 0))
     {
 	int off = (int)(current_ScreenLine - ScreenLines);
+	int max_off = off + screen_Columns;
 	int skip = 0;
 
 	if (wp->w_p_nu && wp->w_p_rnu)
@@ -836,6 +837,10 @@ wlv_screen_line(win_T *wp, winlinevars_T *wlv, int negative_width)
 
 	for (int i = 0; i < 3 && i + skip < wp->w_width; ++i)
 	{
+	    if ((*mb_off2cells)(off, max_off) > 1)
+		// When the first half of a double-width character is
+		// overwritten, change the second half to a space.
+		ScreenLines[off + 1] = ' ';
 	    ScreenLines[off] = '<';
 	    if (enc_utf8)
 		ScreenLinesUC[off] = 0;

src/testdir/dumps/Test_smooth_marker_over_double_width_1.dump

@@ -0,0 +1,6 @@
+>a+0&#ffffff0@39
+|口*&@9| +&@19
+|~+0#4040ff13&| @38
+|~| @38
+|~| @38
+| +0#0000000&@21|1|,|1| @10|A|l@1| 

src/testdir/dumps/Test_smooth_marker_over_double_width_2.dump

@@ -0,0 +1,6 @@
+|<+0#4040ff13#ffffff0@2| +0#0000000&|口*&@6>口| +&@19
+|~+0#4040ff13&| @38
+|~| @38
+|~| @38
+|~| @38
+| +0#0000000&@21|1|,|6|8|-|5|9| @6|A|l@1| 

src/testdir/test_scroll_opt.vim

@@ -399,6 +399,64 @@ func Test_smoothscroll_long_line_showbreak()
   call StopVimInTerminal(buf)
 endfunc
 
+" Check that 'smoothscroll' marker is drawn over double-width char correctly.
+" Run with multiple encodings.
+func Test_smoothscroll_marker_over_double_width()
+  " Run this in a separate Vim instance to avoid messing up.
+  let after =<< trim [CODE]
+    scriptencoding utf-8
+    call setline(1, 'a'->repeat(&columns) .. '口'->repeat(10))
+    setlocal smoothscroll
+    redraw
+    exe "norm \<C-E>"
+    redraw
+    " Check the chars one by one. Don't check the whole line concatenated.
+    call assert_equal('<', screenstring(1, 1))
+    call assert_equal('<', screenstring(1, 2))
+    call assert_equal('<', screenstring(1, 3))
+    call assert_equal(' ', screenstring(1, 4))
+    call assert_equal('口', screenstring(1, 5))
+    call assert_equal('口', screenstring(1, 7))
+    call assert_equal('口', screenstring(1, 9))
+    call assert_equal('口', screenstring(1, 11))
+    call assert_equal('口', screenstring(1, 13))
+    call assert_equal('口', screenstring(1, 15))
+    call writefile(v:errors, 'Xresult')
+    qall!
+  [CODE]
+
+  let encodings = ['utf-8', 'cp932', 'cp936', 'cp949', 'cp950']
+  if !has('win32')
+    let encodings += ['euc-jp']
+  endif
+  for enc in encodings
+    let msg = 'enc=' .. enc
+    if RunVim([], after, $'--clean --cmd "set encoding={enc}"')
+      call assert_equal([], readfile('Xresult'), msg)
+    endif
+    call delete('Xresult')
+  endfor
+endfunc
+
+" Same as the test above, but check the text actually shown on screen.
+" Only run with UTF-8 encoding.
+func Test_smoothscroll_marker_over_double_width_dump()
+  CheckScreendump
+
+  let lines =<< trim END
+    call setline(1, 'a'->repeat(&columns) .. '口'->repeat(10))
+    setlocal smoothscroll
+  END
+  call writefile(lines, 'XSmoothMarkerOverDoubleWidth', 'D')
+  let buf = RunVimInTerminal('-S XSmoothMarkerOverDoubleWidth', #{rows: 6, cols: 40})
+  call VerifyScreenDump(buf, 'Test_smooth_marker_over_double_width_1', {})
+
+  call term_sendkeys(buf, "\<C-E>")
+  call VerifyScreenDump(buf, 'Test_smooth_marker_over_double_width_2', {})
+
+  call StopVimInTerminal(buf)
+endfunc
+
 func s:check_col_calc(win_col, win_line, buf_col)
   call assert_equal(a:win_col, wincol())
   call assert_equal(a:win_line, winline())

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1602,
 /**/
     1601,
 /**/