vim-patch:9.0.1602: stray character visible if marker on top of double-wide char (#23897)

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 vim/vim#12469)

ecb87dd7d3

src/nvim/grid.c

@@ -500,8 +500,6 @@ static int grid_char_needs_redraw(ScreenGrid *grid, size_t off_from, size_t off_
 void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int endcol, int clear_width,
                       int rlflag, win_T *wp, int bg_attr, bool wrap)
 {
-  size_t max_off_from;
-  size_t max_off_to;
   int col = 0;
   bool redraw_next;                         // redraw_this for next character
   bool clear_next = false;
@@ -519,6 +517,7 @@ void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int endcol, int cle
     endcol = grid->cols;
   }
 
+  const size_t max_off_from = (size_t)grid->cols;
   grid_adjust(&grid, &row, &coloff);
 
   // Safety check. Avoids clang warnings down the call stack.
@@ -529,8 +528,7 @@ void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int endcol, int cle
 
   size_t off_from = 0;
   size_t off_to = grid->line_offset[row] + (size_t)coloff;
-  max_off_from = linebuf_size;
-  max_off_to = grid->line_offset[row] + (size_t)grid->cols;
+  const size_t max_off_to = grid->line_offset[row] + (size_t)grid->cols;
 
   // Take care of putting "<<<" on the first line for 'smoothscroll'.
   if (topline && wp->w_skipcol > 0
@@ -538,18 +536,23 @@ void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int endcol, int cle
       && *get_showbreak_value(wp) == NUL
       // do not overwrite the 'listchars' "precedes" text with "<<<"
       && !(wp->w_p_list && wp->w_p_lcs_chars.prec != 0)) {
-    int off = 0;
-    int skip = 0;
+    size_t off = 0;
+    size_t skip = 0;
     if (wp->w_p_nu && wp->w_p_rnu) {
       // do not overwrite the line number, change "123 text" to
       // "123>>>xt".
-      while (skip < wp->w_width_inner && ascii_isdigit(*linebuf_char[off])) {
+      while (skip < max_off_from && ascii_isdigit(*linebuf_char[off])) {
         off++;
         skip++;
       }
     }
 
-    for (int i = 0; i < 3 && i + skip < wp->w_width_inner; i++) {
+    for (size_t i = 0; i < 3 && i + skip < max_off_from; i++) {
+      if (line_off2cells(linebuf_char, off, max_off_from) > 1) {
+        // When the first half of a double-width character is
+        // overwritten, change the second half to a space.
+        schar_from_ascii(linebuf_char[off + 1], ' ');
+      }
       schar_from_ascii(linebuf_char[off], '<');
       linebuf_attr[off] = HL_ATTR(HLF_AT);
       off++;

test/functional/legacy/scroll_opt_spec.lua

@@ -667,6 +667,32 @@ describe('smoothscroll', function()
     screen:expect(s1)
   end)
 
+  -- oldtest: Test_smoothscroll_marker_over_double_width_dump()
+  it('marker is drawn over double-width char correctly', function()
+    screen:try_resize(40, 6)
+    exec([[
+      call setline(1, 'a'->repeat(&columns) .. '口'->repeat(10))
+      setlocal smoothscroll
+    ]])
+    screen:expect([[
+      ^aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa|
+      口口口口口口口口口口                    |
+      ~                                       |
+      ~                                       |
+      ~                                       |
+                                              |
+    ]])
+    feed('<C-E>')
+    screen:expect([[
+      <<< 口口口口口口口^口                    |
+      ~                                       |
+      ~                                       |
+      ~                                       |
+      ~                                       |
+                                              |
+    ]])
+  end)
+
   -- oldtest: Test_smoothscroll_zero_width()
   it("does not divide by zero with a narrow window", function()
     screen:try_resize(12, 2)

test/old/testdir/test_scroll_opt.vim

@@ -399,6 +399,67 @@ 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
+  if has('nvim')
+    let encodings = ['utf-8']
+  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())