Tweak progress bar docs.

There is no good reason as to why the progress bar can't be set back to its indeterminate state (and you could actually do it with code) so this removes the docstring that says that a progress bar can't go back to its indeterminate state.

Related issue: #3268
Related Discord message: https://discord.com/channels/1026214085173461072/1033754296224841768/1149742624002023594

CHANGELOG.md

@@ -31,6 +31,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Breaking change: Widget.notify and App.notify now return None https://github.com/Textualize/textual/pull/3275
 - App.unnotify is now private (renamed to App._unnotify) https://github.com/Textualize/textual/pull/3275
 - `Markdown.load` will now attempt to scroll to a related heading if an anchor is provided https://github.com/Textualize/textual/pull/3244
+- `ProgressBar` explicitly supports being set back to its indeterminate state https://github.com/Textualize/textual/pull/3286
 
 ## [0.36.0] - 2023-09-05
 

src/textual/widgets/_progress_bar.py

@@ -276,7 +276,6 @@ class ProgressBar(Widget, can_focus=False):
     """The total number of steps associated with this progress bar, when known.
 
     The value `None` will render an indeterminate progress bar.
-    Once `total` is set to a numerical value, it cannot be set back to `None`.
     """
     percentage: reactive[float | None] = reactive[Optional[float]](None)
     """The percentage of progress that has been completed.
@@ -398,6 +397,7 @@ def advance(self, advance: float = 1) -> None:
             ```py
             progress_bar.advance(10)  # Advance 10 steps.
             ```
+
         Args:
             advance: Number of steps to advance progress by.
         """

tests/test_progress_bar.py

@@ -78,9 +78,6 @@ def test_update_total():
     pb.update(total=1000)
     assert pb.total == 1000
 
-    pb.update(total=None)
-    assert pb.total == 1000
-
     pb.update(total=100)
     assert pb.total == 100
 
@@ -119,6 +116,15 @@ def test_update():
     assert pb.progress == 50
 
 
+def test_go_back_to_indeterminate():
+    pb = ProgressBar()
+
+    pb.total = 100
+    assert pb.percentage == 0
+    pb.total = None
+    assert pb.percentage is None
+
+
 @pytest.mark.parametrize(
     ["show_bar", "show_percentage", "show_eta"],
     [