Two issues kept the close button unusable:
- The launcher-row close button lacked flex_shrink_0, so on narrow
sidebars the title swallowed the 16 px hit target before flex laid
it out.
- The handler did select_tab(close_id) → close_active_tab(); if the
newly-selected tab routed through split-view close logic the call
silently no-op'd against the user's intent.
Add flex_shrink_0 on both launcher and tab close buttons, and route
the click through a new close_tab_by_id helper that calls
BrowserCore::close_tab(tab_id) directly.
ZacharyZhang-NY