Set the first Collapsible title once and keep label span changes

src/textual/widgets/_collapsible.py

@@ -51,7 +51,7 @@ class CollapsibleTitle(Static, can_focus=True):
     """
 
     collapsed = reactive(True)
-    label: reactive[ContentText] = reactive(Content("Toggle"))
+    label: reactive[ContentText] = reactive(Content("Toggle"), always_update=True)
 
     def __init__(
         self,
@@ -64,8 +64,11 @@ def __init__(
         super().__init__()
         self.collapsed_symbol = collapsed_symbol
         self.expanded_symbol = expanded_symbol
-        self.label = Content.from_text(label)
-        self.collapsed = collapsed
+        # Both inputs belong to the initial title. Do not render reactive
+        # defaults or an intermediate collapsed symbol before acquiring them.
+        self.set_reactive(CollapsibleTitle.label, Content.from_text(label))
+        self.set_reactive(CollapsibleTitle.collapsed, collapsed)
+        self._update_label()
 
     class Toggle(Message):
         """Request toggle."""

tests/test_collapsible.py

@@ -2,12 +2,38 @@
 
 from textual import on
 from textual.app import App, ComposeResult
+from textual.content import Content
 from textual.widgets import Collapsible, Label
 from textual.widgets._collapsible import CollapsibleTitle
 
 COLLAPSED_CLASS = "-collapsed"
 
 
+async def test_title_initial_content_and_same_text_style():
+    title = CollapsibleTitle(
+        label=Content.from_markup("[red]original[/red]"),
+        collapsed_symbol="+",
+        expanded_symbol="---",
+        collapsed=False,
+    )
+    assert title.content.is_same(Content.assemble("---", " ", Content.from_markup("[red]original[/red]")))
+    app = App()
+    async with app.run_test() as pilot:
+        await app.mount(title)
+        await pilot.pause()
+        revision = title._layout_updates
+        title.label = Content.from_markup("[blue]original[/blue]")
+        assert title.content.is_same(
+            Content.assemble("---", " ", Content.from_markup("[blue]original[/blue]"))
+        )
+        assert title._layout_updates == revision
+        title.collapsed = True
+        assert title.content.plain == "+ original"
+        assert title._layout_updates > revision
+        await pilot.pause()
+        assert title.region.width == len("+ original") + 2
+
+
 def get_title(collapsible: Collapsible) -> CollapsibleTitle:
     return collapsible.get_child_by_type(CollapsibleTitle)