floats: fix 'winblend' on top of doublewidth chars.

The interaction between 'winblend' and doublewidth chars in the background
does not look very good. But check no chars get incorrectly placed
at least.

Also check that hidden EndOfBuffer region (from style="minimal") blends
correctly.

3 files changed, 105 insertions, 3 deletions

diff --git a/src/nvim/highlight_defs.h b/src/nvim/highlight_defs.h
index 60f571ff0f..25d859c55d 100644
--- a/src/nvim/highlight_defs.h
+++ b/src/nvim/highlight_defs.h
@@ -36,6 +36,7 @@ typedef struct attr_entry {
   .rgb_sp_color = -1, \
   .cterm_fg_color = 0, \
   .cterm_bg_color = 0, \
+  .hl_blend = -1, \
 }
 
 /// Values for index in highlight_attr[].
diff --git a/src/nvim/ui_compositor.c b/src/nvim/ui_compositor.c
index 858ffbe5bc..d0b21ae591 100644
--- a/src/nvim/ui_compositor.c
+++ b/src/nvim/ui_compositor.c
@@ -344,11 +344,22 @@ static void compose_line(Integer row, Integer startcol, Integer endcol,
 
     // 'pumblend' and 'winblend'
     if (grid->blending) {
-      for (int i = col-(int)startcol; i < until-startcol; i++) {
-        bool thru = strequal((char *)linebuf[i], " ");  // negative space
+      int width;
+      for (int i = col-(int)startcol; i < until-startcol; i += width) {
+        width = 1;
+        // negative space
+        bool thru = strequal((char *)linebuf[i], " ") && bg_line[i][0] != NUL;
+        if (i+1 < endcol-startcol && bg_line[i+1][0] == NUL) {
+          width = 2;
+          thru &= strequal((char *)linebuf[i+1], " ");
+        }
         attrbuf[i] = (sattr_T)hl_blend_attrs(bg_attrs[i], attrbuf[i], &thru);
+        if (width == 2) {
+          attrbuf[i+1] = (sattr_T)hl_blend_attrs(bg_attrs[i+1],
+                                                 attrbuf[i+1], &thru);
+        }
         if (thru) {
-          memcpy(linebuf[i], bg_line[i], sizeof(linebuf[i]));
+          memcpy(linebuf[i], bg_line[i], (size_t)width * sizeof(linebuf[i]));
         }
       }
     }
diff --git a/test/functional/ui/float_spec.lua b/test/functional/ui/float_spec.lua
index 68a23db0a2..8a1758c4a0 100644
--- a/test/functional/ui/float_spec.lua
+++ b/test/functional/ui/float_spec.lua
@@ -4659,6 +4659,96 @@ describe('floating windows', function()
                                                   |
         ]])
       end
+
+      -- The interaction between 'winblend' and doublewidth chars in the background
+      -- does not look very good. But check no chars get incorrectly placed
+      -- at least. Also check invisible EndOfBuffer region blends correctly.
+      meths.buf_set_lines(buf, 0, -1, true, {" x x  x   xx", "  x x  x   x"})
+      win = meths.open_win(buf, false, {relative='editor', width=12, height=3, row=0, col=11, style='minimal'})
+      meths.win_set_option(win, 'winblend', 30)
+      screen:set_default_attr_ids({
+        [1] = {foreground = tonumber('0xb282b2'), background = tonumber('0xffcfff')},
+        [2] = {foreground = Screen.colors.Grey0, background = tonumber('0xffcfff')},
+        [3] = {bold = true, foreground = Screen.colors.Blue1},
+        [4] = {background = tonumber('0xffcfff'), bold = true, foreground = tonumber('0xb282ff')},
+        [5] = {background = Screen.colors.LightMagenta},
+      })
+      if multigrid then
+        screen:expect{grid=[[
+        ## grid 1
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+                                                  |
+        ## grid 2
+          # TODO: 测试字典信息的准确性            |
+          # FIXME: 测试字典信息的准确^性           |
+          {3:~                                       }|
+          {3:~                                       }|
+          {3:~                                       }|
+          {3:~                                       }|
+        ## grid 5
+          {5: x x  x   xx}|
+          {5:  x x  x   x}|
+          {5:            }|
+        ]], float_pos={
+          [5] = { {
+              id = 1003
+            }, "NW", 1, 0, 11, true }
+        }}
+      else
+        screen:expect([[
+          # TODO: 测 {2: x x  x}{1:息}{2: xx} 确性            |
+          # FIXME: 测{1:试}{2:x x  x}{1:息}{2: x}准确^性           |
+          {3:~          }{4:            }{3:                 }|
+          {3:~                                       }|
+          {3:~                                       }|
+          {3:~                                       }|
+                                                  |
+        ]])
+      end
+
+      meths.win_set_config(win, {relative='editor', row=0, col=12})
+      if multigrid then
+        screen:expect{grid=[[
+        ## grid 1
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+                                                  |
+        ## grid 2
+          # TODO: 测试字典信息的准确性            |
+          # FIXME: 测试字典信息的准确^性           |
+          {3:~                                       }|
+          {3:~                                       }|
+          {3:~                                       }|
+          {3:~                                       }|
+        ## grid 5
+          {5: x x  x   xx}|
+          {5:  x x  x   x}|
+          {5:            }|
+        ]], float_pos={
+          [5] = { {
+              id = 1003
+            }, "NW", 1, 0, 12, true }
+        }}
+      else
+        screen:expect([[
+          # TODO: 测试{2: x x}{1:信}{2:x }{1:的}{2:xx}确性            |
+          # FIXME: 测 {2:  x x}{1:信}{2:x }{1:的}{2:x} 确^性           |
+          {3:~           }{4:            }{3:                }|
+          {3:~                                       }|
+          {3:~                                       }|
+          {3:~                                       }|
+                                                  |
+        ]])
+      end
     end)
   end