@epriestley, test files in last comment indicate 31 can decompress gzip properly.
please check it.

test:

• test script:
  test.php490 BDownload
• dumped POST:
  test.txt.gz9 KBDownload
• run:

php test.php 31 && tail test.txt
php test.php 30 && tail test.txt

In T10264#160688, @epriestley wrote:

Does changing window to 31 actually fix it for you?

Still hang...
dumping decompressed result shows input was truncated.

I got the same problem with HEAD of master.
it seems relate to clone a git repo with gziped post data by http.

It must inconsistent:

• doc said "at least 8mb to support large file"
• truth is "bigger then 4mb will work"