When (and why) did Warner Bros. get the rights to Mack Sennett shorts?

I noticed that TCM's schedule for tomorrow has the short Happy Times and Jolly Memories, tomorrow morning at about 11:50 AM, or just after Topper Takes a Trip (10:30 AM, 80 min). This particular short is new to me; it's a 1943 short from Warner Bros. looking back at the olden days of silent comedy by showing a whole bunch of clips from Mack Sennett's comedies at Keystone. (But, as the IMDb commenters say, not including Charlie Chaplin; I wondered if he nixed his appearance.) To be honest, I wondered how Warner Bros. got all these clips back in 1943. Keystone had gone defunct in 1935, long after Sennett left. However, a Wikipedia search of Sennett's biography says that many of Sennett's movies that he made after leaving Keystone were in fact distributed by First National. Warner Bros. of course, later bought out First National, and that ould explain how they got all the shorts, but not any of those with Chaplin. You learn something new every day.

Elsewhere in the shorts department, TCM is once again running the King Solomon's Mines promo that's really a promo for Dodge trucks, overnight tonight or very early tomorrow morning at about 4:15 AM. One that I haven't seen before is The Backyard Broadcast, a two-reeler from 1936 coming up a little after 7:30 AM tomorrow. This one seems to be another of the revue of juvenile talent acts, much like the two that aired on Judy Garland's birthday a few weeks back, or Show Kids. Unsurprisingly, I don't recognize any of the child acts here.