fix(api): make nvim_set_hl(ns=0, ...) redraw screen properly

fixes #18160

src/nvim/highlight.c

@@ -303,8 +303,6 @@ int hl_get_ui_attr(int ns_id, int idx, int final_id, bool optional)
     if (pum_drawn()) {
       must_redraw_pum = true;
     }
-  } else if (idx == HLF_MSG && ns_id == -1) {
-    msg_grid.blending = attrs.hl_blend > -1;
   }
 
   if (optional && !available) {

src/nvim/highlight_group.c

@@ -698,7 +698,7 @@ void set_hl_group(int id, HlAttrs attrs, Dict(highlight) *dict, int link_id)
       g->sg_deflink_sctx = current_sctx;
       g->sg_deflink_sctx.sc_lnum += SOURCING_LNUM;
     }
-    return;
+    goto update;
   }
 
   g->sg_cleared = false;
@@ -753,6 +753,12 @@ void set_hl_group(int id, HlAttrs attrs, Dict(highlight) *dict, int link_id)
       ui_mode_info_set();
     }
   }
+
+update:
+  if (!updating_screen) {
+    redraw_all_later(NOT_VALID);
+  }
+  need_highlight_changed = true;
 }
 
 /// Handle ":highlight" command
@@ -1942,6 +1948,8 @@ void highlight_changed(void)
     if (highlight_attr[hlf] != highlight_attr_last[hlf]) {
       if (hlf == HLF_MSG) {
         clear_cmdline = true;
+        HlAttrs attrs = syn_attr2entry(highlight_attr[hlf]);
+        msg_grid.blending = attrs.hl_blend > -1;
       }
       ui_call_hl_group_set(cstr_as_string((char *)hlf_names[hlf]),
                            highlight_attr[hlf]);

test/functional/ui/highlight_spec.lua

@@ -2335,6 +2335,7 @@ describe('highlight namespaces', function()
       [5] = {background = Screen.colors.Magenta4, foreground = Screen.colors.Crimson};
       [6] = {bold = true, reverse = true};
       [7] = {reverse = true};
+      [8] = {foreground = Screen.colors.Gray20};
     }
 
     ns1 = meths.create_namespace 'grungy'
@@ -2425,4 +2426,33 @@ describe('highlight namespaces', function()
                                |
     ]]}
   end)
+
+  it('redraws correctly when ns=0', function()
+    screen:expect{grid=[[
+      ^                         |
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+      {1:~                        }|
+                               |
+    ]]}
+
+    meths.set_hl(0, 'EndOfBuffer', {fg='#333333'})
+    screen:expect{grid=[[
+      ^                         |
+      {8:~                        }|
+      {8:~                        }|
+      {8:~                        }|
+      {8:~                        }|
+      {8:~                        }|
+      {8:~                        }|
+      {8:~                        }|
+      {8:~                        }|
+                               |
+    ]]}
+  end)
 end)