Desktop: expanding the PR page to full width leaves it at half width with empty space on the right #705

Seen by the #698 worker on main 5b8ebbb (desktop, English, light, a docked PR page for a linked Azure DevOps PR): expanding the PR page to full width left it at about half the window's width with an empty area to its right, unchanged after 2 s; going back to the docked layout rendered normally. Not reproduced deliberately and not localized. Owner: the PR page's expanded layout in crates/ui/src/pull_requests/detail.rs and the right panel's expand state (shell.rs, after #691). Repro: link any PR, open the PR page docked, press Expand.