When a merge empties the root, the tree gets shorter. Today you collapse a root that has lost its last separator into its single remaining child, the mirror image of growing a new root - and the only way a B+Tree loses height.
When the root is an internal node left with no keys and a single child, make that child the new root, reducing the tree's height.
Deletes only ever shorten a B+Tree at the root, exactly as inserts only ever lengthen it there. When a merge removes the root’s last separator, the root is an internal node with a single child and no way left to route - so it is discarded and that lone child becomes the new root. The whole tree drops a level, and because it happens at the very top, every leaf stays at the same depth, just one shallower.
The root gets one exemption the rest of the tree does not: a root leaf is allowed to fall all the way to empty without collapsing, because an empty tree is a perfectly valid tree - it is just where you started. With this, delete is complete: borrow when you can, merge when you cannot, cascade the repair upward, and collapse the root when it empties. The tree stays balanced and every leaf equidistant from the root through any mix of inserts and deletes.
// after handling the root's children, if the root is internal and has// 0 keys (so exactly 1 child), set t.root = that child's id and free// the old root page. the tree just lost a level.// a root LEAF is never collapsed - an empty root leaf is a valid tree.