Fix overlapping mobile thread list More menus

[brsbl]

Human comments

What was wrong

On mobile, opening a hidden thread section from More stacked two drawers. The section also drew a divider above Add to list.

What changed

• Hidden sections replace More within one mobile drawer, with Back to return to the list.
• Picking a thread keeps the section visible while the drawer closes; reopening More starts at the list.
• The section's threads scroll while Back and Add to list stay pinned, without a top divider.

How you verified

• Regression tests cover the one-drawer flow, the closing drawer keeping its section, and reopening at the More list.
• Chrome for Testing through BB Browser Automation, branch web app, 390×844 at 2× with touch emulation, 33-thread hidden section: one drawer; thread scroller 649px tall with 1174px content, scrolling by wheel and touch; Back and Add to list stayed fixed at every scroll position.
• Desktop 1440×900 hover submenu still lists the section's threads with Add to list at the bottom.
• iOS Simulator Safari remains unrun; this checkout's host has no Simulator.

Before: 9bfe862b35 (merge base) · After: 74c5b77f97 · Captured at 2×

Screen / state	Viewport	Before	After
More / long hidden section open	Mobile · 390×844

The row below compares against the previous PR head 0d1475e8a5, where review found this regression; it did not exist on the merge base in this form.

Screen / state	Viewport	Before (0d1475e8a5)	After (74c5b77f97)
Long hidden section, list at top	Mobile · 390×844

BB-Thread-ID: thr_h6cfedfsgn

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