fix(options): using :set fillchars should clear local value

src/nvim/option.c

@@ -2665,14 +2665,19 @@ ambw_end:
     }
   } else if (varp == &curwin->w_p_lcs) {  // local 'listchars'
     errmsg = set_chars_option(curwin, varp, true);
-  } else if (varp == &p_fcs) {  // 'fillchars'
+  } else if (varp == &p_fcs) {  // global 'fillchars'
     errmsg = set_chars_option(curwin, varp, false);
-    if (!errmsg) {
+    if (errmsg == NULL) {
+      // The current window is set to use the global 'fillchars' value.
+      // So clear the window-local value.
+      if (!(opt_flags & OPT_GLOBAL)) {
+        clear_string_option(&curwin->w_p_fcs);
+      }
       FOR_ALL_TAB_WINDOWS(tp, wp) {
         set_chars_option(wp, &wp->w_p_fcs, true);
       }
-    }
       redraw_all_later(NOT_VALID);
+    }
   } else if (varp == &curwin->w_p_fcs) {  // local 'fillchars'
     errmsg = set_chars_option(curwin, varp, true);
   } else if (varp == &p_cedit) {  // 'cedit'

test/functional/options/chars_spec.lua

@@ -67,6 +67,7 @@ describe("'fillchars'", function()
       shouldfail('eob:xy') -- two ascii chars
       shouldfail('eob:\255', 'eob:<ff>') -- invalid UTF-8
     end)
+  end)
   it('has global value', function()
     screen:try_resize(50, 5)
     insert("foo\nbar")
@@ -82,7 +83,7 @@ describe("'fillchars'", function()
                                                         |
     ]])
   end)
-    it('has local window value', function()
+  it('has window-local value', function()
     screen:try_resize(50, 5)
     insert("foo\nbar")
     command('set laststatus=0')
@@ -97,6 +98,21 @@ describe("'fillchars'", function()
                                                         |
     ]])
   end)
+  it('using :set clears window-local value', function()
+    screen:try_resize(50, 5)
+    insert("foo\nbar")
+    command('set laststatus=0')
+    command('setl fillchars=fold:x')
+    command('1,2fold')
+    command('vsplit')
+    command('set fillchars&')
+    screen:expect([[
+      ^+--  2 lines: foo········│+--  2 lines: fooxxxxxxx|
+      ~                        │~                       |
+      ~                        │~                       |
+      ~                        │~                       |
+                                                        |
+    ]])
   end)
 end)