test sub titles

Author: willmcgugan

src/textual/app.py

@@ -977,7 +977,11 @@ def format_title(self, title: str, sub_title: str) -> Content:
         title_content = Content(title)
         sub_title_content = Content(sub_title)
         if sub_title_content:
-            return Content.assemble(title_content, " — ", sub_title_content)
+            return Content.assemble(
+                title_content,
+                (" — ", "dim"),
+                sub_title_content.stylize("dim"),
+            )
         else:
             return title_content
 

tests/snapshot_tests/__snapshots__/test_snapshots/test_header_format.svg

@@ -4610,3 +4610,19 @@ def compose(self) -> ComposeResult:
             yield TextArea(placeholder="Your text here")
 
     assert snap_compare(TextApp())
+
+
+def test_header_format(snap_compare):
+    """Test title and sub-title are formatted as expected.
+
+    You should see "Title - Sub-title" in the header. Where sub-title is dimmed.
+    """
+
+    class HeaderApp(App):
+        TITLE = "Title"
+        SUB_TITLE = "Sub-title"
+
+        def compose(self) -> ComposeResult:
+            yield Header()
+
+    assert snap_compare(HeaderApp())