bpo-15786: Fix IDLE autocomplete return problem. by terryjreedy · Pull Request #2198 · python/cpython

Before, <return> would not, for instance, complete 're.c' to 're.compile' even with 'compile' highlighted.  Now it does.  Before, <return> was inserted into text, which in Shell meant compile() and possibly execute.  Now cursor is left after completion.
(cherry picked from commit 32fd874)

Before,  Enter would not, for instance, complete 're.c' to 're.compile' even with 'compile' highlighted.  Now it does.  Before, '\n' was inserted into text, which in Shell meant compile() and possibly execute.  Now cursor is left after completion.
(cherry picked from commit 32fd874)