Chris Brown is all good with Drake doing the midnight jockey ride with Rihanna -- even though she caused MAJOR BEEF between them for years.

Sources close to Chris tell TMZ ... the singer is copacetic over the duo rekindling their romance.  Chris has his mind on his money ... focusing on making a comeback.

You'll recall ... all the quien es mas macho over Rihanna triggered an all-out war at WIP nightclub in NYC back in 2012 ... when the Brown/Drake posses got into a bottle throwing fight.

TMZ broke the story ... doctors at the rehab facility where he just finished a 90-day stint have diagnosed the singer with bipolar disorder. 

Maybe the meds are working.