Merging two free blocks makes a bigger block that belongs in a different bin. Today you make coalescing bin-aware, so a merged block is removed from its old bin and filed under its new, larger size class.
Ensure coalescing removes merged neighbours from their bins and files the result under its new size class.
Coalescing and segregation interact in one specific way: when two free blocks merge, the result is larger, so it belongs in a different bin than either piece did. If you forget this, a merged block ends up filed under a size class that no longer matches its size, and allocation searching by class will never find it - a subtle leak.
The fix is small but essential: whenever coalescing pulls a neighbour out to merge it, remove it from its bin (the one matching its old size), and insert the merged block by its new size. Two size-32 blocks (bin 1) that merge become a size-64 block (bin 4), and bin 1 must be left empty. This is the kind of invariant that is easy to break and hard to notice by eye - which is exactly why the next lesson builds a checker that verifies every free block sits in the bin it should.
// coalescing already unlinks each free neighbour before merging.// the only change: unlink from the neighbour's size-class bin, and// insert the merged block by its NEW size:a.unlinkFrom(classOf(nsize), nextOff) // successora.unlinkFrom(classOf(psize), prev) // predecessor// ... size grows ...a.insertInto(classOf(size), off) // merged block, new class