I asked the Codeforces about recent progress. That's what they told me:

Polygon:

• true language autodetection (Polygon tries to compile a source using different dialects),

• you can upload multiple items simultaneously on the tabs Files and Solutions,

• feature to view stderr for invocations,

• Rejudge/Abort links inside invocation page,

In addition, the work on API continues.

Codeforces:

• preview comment when editing;

• filter by problems on hacks page;

• improved choice of t-shirt size;

• the ability to change the handle once in the first 7 days after a registration;

• improved performance of some API methods.