fix missing new widgets

CHANGELOG.md

@@ -15,6 +15,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Tree will no longer scroll the X axis when moving the cursor https://github.com/Textualize/textual/pull/5047
 
+### Fixed
+
+- Fixed widgets occasionally not getting Resize events https://github.com/Textualize/textual/pull/5048
+
 ## [0.80.1] - 2024-09-24
 
 ### Fixed

src/textual/_compositor.py

@@ -309,6 +309,9 @@ def __init__(self) -> None:
         # Mapping of line numbers on to lists of widget and regions
         self._layers_visible: list[list[tuple[Widget, Region, Region]]] | None = None
 
+        # New widgets added between updates
+        self._new_widgets: set[Widget] = set()
+
     def clear(self) -> None:
         """Remove all references to widgets (used when the screen closes)."""
         self._full_map.clear()
@@ -387,7 +390,9 @@ def reflow(self, parent: Widget, size: Size) -> ReflowResult:
         new_widgets = map.keys()
 
         # Newly visible widgets
-        shown_widgets = new_widgets - old_widgets
+        shown_widgets = (new_widgets - old_widgets) | self._new_widgets
+        self._new_widgets.clear()
+
         # Newly hidden widgets
         hidden_widgets = self.widgets - widgets
 
@@ -419,7 +424,6 @@ def reflow(self, parent: Widget, size: Size) -> ReflowResult:
             for widget, (region, *_) in changes
             if (widget in common_widgets and old_map[widget].region[2:] != region[2:])
         }
-
         return ReflowResult(
             hidden=hidden_widgets,
             shown=shown_widgets,
@@ -483,6 +487,8 @@ def full_map(self) -> CompositorMap:
         if self._full_map_invalidated:
             self._full_map_invalidated = False
             map, _widgets = self._arrange_root(self.root, self.size, visible_only=False)
+            # Update any widgets which became visible in the interim
+            self._new_widgets.update(map.keys() - self._full_map.keys())
             self._full_map = map
             self._visible_widgets = None
             self._visible_map = None

tests/snapshot_tests/test_snapshots.py

@@ -9,7 +9,7 @@
 from textual import events
 from textual.app import App, ComposeResult
 from textual.binding import Binding
-from textual.containers import Vertical
+from textual.containers import Vertical, VerticalScroll
 from textual.pilot import Pilot
 from textual.screen import Screen
 from textual.widgets import (
@@ -24,8 +24,7 @@
     OptionList,
     SelectionList,
 )
-from textual.widgets import Switch
-from textual.widgets import Label
+from textual.widgets import ProgressBar, Label, Switch
 from textual.widgets.text_area import BUILTIN_LANGUAGES, Selection, TextAreaTheme
 
 # These paths should be relative to THIS directory.
@@ -1992,3 +1991,31 @@ def on_mount(self) -> None:
 
     app = DisabledApp()
     assert snap_compare(app)
+
+
+def test_missing_new_widgets(snap_compare):
+    """Regression test for https://github.com/Textualize/textual/issues/5024"""
+
+    class MRE(App):
+        BINDINGS = [("z", "toggle_console", "Console")]
+        CSS = """
+        RichLog { border-top: dashed blue; height: 6; }
+        .hidden { display: none; }
+        """
+
+        def compose(self):
+            yield VerticalScroll()
+            yield ProgressBar(
+                classes="hidden"
+            )  # removing or displaying this widget prevents the bug
+            yield Footer()  # clicking "Console" in the footer prevents the bug
+            yield RichLog(classes="hidden")
+
+        def on_ready(self) -> None:
+            self.query_one(RichLog).write("\n".join(f"line #{i}" for i in range(5)))
+
+        def action_toggle_console(self) -> None:
+            self.query_one(RichLog).toggle_class("hidden")
+
+    app = MRE()
+    assert snap_compare(app, press=["space", "space", "z"])