There would still have be some buffer between the amount of space you have left and the size of this heap - even if the OS were changed to treat this specific c

There would still have be some buffer between the amount of space you have left and the size of this heap - even if the OS were changed to treat this specific case as not being out of disk space, it would significantly impact write performance on the disk once it is 'full', because any disk operation would have potentially double the number of operations required compared to being able to write it to empty space on the disk