Gate finding 2 (Medium, quality r2): fold1 only derived "follow the
remembered group's chat" inside $focusedTreePaneId and never wrote it
back to $lastContentPane. With the sidebar focused, revealing or
dragging a new tile into the remembered group was followed, but a
preview later covering that tile snapped focus back to the stale
remembered pane, so the Files rail and statusbar jumped to the old chat.
rememberContentPane now records the remembered group's chat when it
changes under chrome focus, which lets the computed collapse to "the
remembered pane if it still exists, else the workspace group's tab"
(drops the nested ternary and the non-null assertion). The existing
follow assertion is extended with a preview covering the followed chat.
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