Fix placeholder color cycling.

CHANGELOG.md

@@ -5,6 +5,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## Unreleased
+
+### Changed
+
+- `Placeholder` now sets its color cycle per app https://github.com/Textualize/textual/issues/2590
 
 ## [0.25.0] - 2023-05-17
 

src/textual/widgets/_placeholder.py

@@ -3,10 +3,14 @@
 from __future__ import annotations
 
 from itertools import cycle
+from typing import Iterator
+from weakref import WeakKeyDictionary
 
 from rich.console import RenderableType
 from typing_extensions import Literal, Self
 
+from textual.app import App
+
 from .. import events
 from ..css._error_tools import friendly_list
 from ..reactive import Reactive, reactive
@@ -72,18 +76,13 @@ class Placeholder(Widget):
     """
 
     # Consecutive placeholders get assigned consecutive colors.
-    _COLORS = cycle(_PLACEHOLDER_BACKGROUND_COLORS)
+    _COLORS: WeakKeyDictionary[App, Iterator[str]] = WeakKeyDictionary()
     _SIZE_RENDER_TEMPLATE = "[b]{} x {}[/b]"
 
     variant: Reactive[PlaceholderVariant] = reactive[PlaceholderVariant]("default")
 
     _renderables: dict[PlaceholderVariant, str]
 
-    @classmethod
-    def reset_color_cycle(cls) -> None:
-        """Reset the placeholder background color cycle."""
-        cls._COLORS = cycle(_PLACEHOLDER_BACKGROUND_COLORS)
-
     def __init__(
         self,
         label: str | None = None,
@@ -113,8 +112,6 @@ class Placeholder(Widget):
 
         super().__init__(name=name, id=id, classes=classes)
 
-        self.styles.background = f"{next(Placeholder._COLORS)} 50%"
-
         self.variant = self.validate_variant(variant)
         """The current variant of the placeholder."""
 
@@ -123,6 +120,13 @@ class Placeholder(Widget):
         while next(self._variants_cycle) != self.variant:
             pass
 
+    def on_mount(self) -> None:
+        """Set the color for this placeholder."""
+        colors = Placeholder._COLORS.setdefault(
+            self.app, cycle(_PLACEHOLDER_BACKGROUND_COLORS)
+        )
+        self.styles.background = f"{next(colors)} 50%"
+
     def render(self) -> RenderableType:
         """Render the placeholder.