Splitting a pane right from the default layout tore down the existing
pane and rebuilt it, losing the composer, terminal, and scroll state in
it.
Both split reducers derived the parent group from `parentPath.length`,
but `targetPath.slice(0, -1)` is empty for two different situations: the
pane is the whole tree, and the pane is a direct child of the root
group. The second case was read as parentless, so the same-direction
fast path never ran at root level and every split wrapped the pane in a
fresh group. That reparents the pane, and the renderer reconciles panes
positionally, so React unmounted the subtree. The workspace already
starts as a horizontal group holding the main pane and the hidden
explorer pane — the group was there all along, ready to take another
child.
`findParentGroup` now asks `targetPath`, so a same-direction split
appends a sibling into the group that is already running in that axis.
Perpendicular splits still nest, because changing axis needs a group.
The old test asserted the nesting as intended behaviour, which is how
this survived. It now asserts flat insertion, alongside the invariant
that matters: a same-direction split never changes an existing pane's
render path. Node id sequences shifted in three tests because the first
split no longer spends a group id, and four user splits now fit inside
the depth cap where the fourth used to be refused.
Mohamed Boudra
5997cc1c17