added test

Author: Frank

tests/text_area/test_scrollbar_alignment.py

@@ -0,0 +1,73 @@
+from __future__ import annotations
+
+from textual.app import App, ComposeResult
+from textual.widgets import TextArea, Tree, OptionList, Static
+from textual.containers import VerticalScroll
+
+
+class _ScrollbarDemo(App[None]):
+	"""Minimal app to exercise vertical scrollbar alignment.
+
+	The widgets chosen mirror the original report (TextArea, Tree, OptionList, VerticalScroll)
+	and are intentionally given enough content to show a vertical scrollbar.
+	"""
+
+	CSS = """
+	Screen > * {
+		width: 1fr;
+		height: 1fr;
+		border: none;
+		background: $surface;
+	}
+	"""
+
+	def compose(self) -> ComposeResult:
+		yield TextArea(id="ta")
+		yield Tree("", id="tree")
+		yield OptionList(id="ol")
+		yield VerticalScroll(id="vs")
+
+	def on_mount(self) -> None:
+		# TextArea content
+		self.query_one(TextArea).load_text("\n".join(f"Line {n}" for n in range(200)))
+
+		# Tree content
+		tree = self.query_one(Tree)
+		tree.root.expand()
+		for n in range(200):
+			tree.root.add(f"Node {n}")
+
+		# OptionList content
+		self.query_one(OptionList).add_options(f"Option {n}" for n in range(200))
+
+		# VerticalScroll content
+		self.query_one("#vs").mount_all(*(Static(f"Item {n}") for n in range(200)))
+
+
+async def test_textarea_scrollbar_aligns_with_peers():
+	"""TextArea's vertical scrollbar should align with Tree, OptionList, and VerticalScroll.
+
+	We check the right-most edge of each widget's region; if they match, the scrollbars 
+	are visually aligned to the same gutter.
+	"""
+	app = _ScrollbarDemo()
+	async with app.run_test() as pilot:
+		# Let layout settle
+		await pilot.pause(0.05)
+
+		ta = app.query_one("#ta")
+		tree = app.query_one("#tree")
+		ol = app.query_one("#ol")
+		vs = app.query_one("#vs")
+
+		# Sanity: all should be showing vertical scrollbars given the content size
+		assert ta.show_vertical_scrollbar, "TextArea should show vertical scrollbar"
+		assert tree.show_vertical_scrollbar, "Tree should show vertical scrollbar"
+		assert ol.show_vertical_scrollbar, "OptionList should show vertical scrollbar"
+		assert vs.show_vertical_scrollbar, "VerticalScroll should show vertical scrollbar"
+
+		right_edges = [w.region.right for w in (ta, tree, ol, vs)]
+		assert len(set(right_edges)) == 1, (
+			"Expected aligned scrollbars: right edges differ "
+			f"{right_edges}"
+		)