fix(ui): trigger TabNewEntered after split new tab

src/nvim/window.c

@@ -4261,7 +4261,11 @@ int may_open_tabpage(void)
 
   cmdmod.cmod_tab = 0;         // reset it to avoid doing it twice
   postponed_split_tab = 0;
-  return win_new_tabpage(n, NULL);
+  int status = win_new_tabpage(n, NULL);
+  if (status == OK) {
+    apply_autocmds(EVENT_TABNEWENTERED, NULL, NULL, false, curbuf);
+  }
+  return status;
 }
 
 // Create up to "maxcount" tabpages with empty windows.

test/functional/autocmd/tabnewentered_spec.lua

@@ -35,6 +35,14 @@ describe('TabNewEntered', function()
         eq('entered', nvim('exec', 'execute "normal \\<C-W>T"', true))
       end)
     end)
+    describe('with tab split #4334', function()
+      it('works when create a tab by using tab split command', function()
+        clear()
+        nvim('command', 'au! TabNewEntered * let b:entered = "entered"')
+        nvim('command', 'tab split')
+        eq('entered', nvim('exec', 'echo b:entered', true))
+      end)
+    end)
   end)
 end)