How does a leaf node split in innodb?

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database

If the records are inserted in ascending order(usually primary key is auto-increment),so each split will cause the old leaf half-full.Since the records are inserted in ascending order,so the later records will be insert into the new leaf node until the next split.Old leaf node will keep half-full state.
Actually a logical node is equal to a physical page in innodb.So if the half-full page cause a waste of storage space?

What I have tried:

read reference:
b+tree split:Wiki B+ tree
innodb b+ tree index structure:B+Tree index structures in InnoDB - Jeremy Cole