Scrollbars do not reflect CSS changes immediately

[xavierog]

Context

Checked closed issues? Yes.

Expectations

The scrollbar should reflect CSS changes (scrollbar-color) immediately.

Current behaviour

The scrollbar does not change until one scrolls or mouse-enters the scrollbar.
The MRE below reflects the issue.

#!/usr/bin/env python3
from textual.app import App, ComposeResult
from textual.containers import ScrollableContainer
from textual.widgets import Static, Footer

class MyBoringWidget(Static, can_focus=True):
	DEFAULT_CSS = "MyBoringWidget { height: 90%; background: $primary; border: tall yellow; }"

class ScrollMRE(App):
	CSS = """
	ScrollableContainer {
		scrollbar-color: ansi_bright_red;
		scrollbar-color-active: ansi_bright_red;
		scrollbar-color-hover: ansi_bright_red;
	}
	ScrollableContainer.foo {
		scrollbar-color: ansi_bright_green;
		scrollbar-color-active: ansi_bright_green;
		scrollbar-color-hover: ansi_bright_green;
	}
	Static { text-align: center; }
	"""
	BINDINGS = [
		('ctrl+f', 'toggle', 'Toggle scrollbar handle color'),
		('ctrl+w', 'workaround', 'Toggle workaround'),
	]
	def action_workaround(self):
		self.workaround = not self.workaround

	def action_toggle(self):
		self.scrollable_container.toggle_class('foo')
		if self.workaround:
			scrollbar = self.scrollable_container.vertical_scrollbar
			value = scrollbar.mouse_over
			scrollbar.mouse_over = not value
			scrollbar.mouse_over = value

	def compose(self) -> ComposeResult:
		self.workaround = False
		self.scrollable_container = ScrollableContainer()
		with self.scrollable_container:
			yield Static('Hit Ctrl+f. Expectations: the scrollbar handle should change color immediately')
			yield MyBoringWidget()
			yield Static('Actual behaviour: it does not change until one scrolls')
			yield MyBoringWidget()
			yield Static('Hit Ctrl+w to enable the workwaround')
		yield Footer()

if __name__ == "__main__":
	app = ScrollMRE()
	app.run()

Textual Diagnostics

Versions

Name	Value
Textual	0.56.4
Rich	13.7.1

Python

Name	Value
Version	3.11.9
Implementation	CPython
Compiler	GCC 13.2.0
Executable	~/whatever/venv/bin/python3

Operating System

Name	Value
System	Linux
Release	6.7.9-amd64
Version	#1 SMP PREEMPT_DYNAMIC Debian 6.7.9-2 (2024-03-13)

Terminal

Name	Value
Terminal Application	tmux (3.4)
TERM	tmux-256color
COLORTERM	truecolor
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=255, height=43
legacy_windows	False
min_width	1
max_width	255
is_terminal	False
encoding	utf-8
max_height	43
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None

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