[BUG] ListBox wrapped in AttrMap is not scrollable with ScrollBar

[rsekman]

Description:

Wrapping a ListBox in an AttrMap should be transparent for operations such as adding a ScrollBar, but throws because the AttrMap is not realised to be scrollable.

Affected versions (if applicable)

• master branch (7300acf)
• Latest stable version from pypi
• All versions since 2.5.0

Steps to reproduce (if applicable)

import urwid as uw
w = uw.ScrollBar(uw.AttrMap(uw.ListBox([]), {}))

Expected/actual outcome

Expected behaviour: w can be scrolled as if it were not wrapped in an AttrMap.
Actual behaviour:

Traceback (most recent call last):
  File "/home/koyomi/Sandbox/test_attrmap.py", line 8, in <module>
    w = uw.ScrollBar(am)
        ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/site-packages/urwid/widget/scrollable.py", line 486, in __init__
    raise TypeError(f"Not a scrollable widget: {widget!r}")
TypeError: Not a scrollable widget: <AttrMap selectable box widget <ListBox selectable box widget> attr_map={}>

Cause

While Scrollable is aware of wrappers and descends into them to see if the original widgets are scrollable

urwid/urwid/widget/scrollable.py

Lines 615 to 625 in 7300acf

	def orig_iter(w: Widget) -> Iterator[Widget]:
	while hasattr(w, "original_widget"):
	w = w.original_widget
	yield w
	yield w
	w = self
	for w in orig_iter(self):
	if isinstance(w, SupportsScroll):
	return w

ScrollBar is not and does not

urwid/urwid/widget/scrollable.py

Lines 484 to 485 in 7300acf

	if not isinstance(widget, SupportsScroll):
	raise TypeError(f"Not a scrollable widget: {widget!r}")

Fix

Make the check in ScrollBar analogous to the check in Scrollable.

[mfncooper]

Note that this bug also affects the tour.py example that ships with Urwid. This example thus provides a nice test case for the fix in #879, built right into the Urwid package. It would be good to verify that the examples work before releasing.