When you launch the game, the game output log comes up (if you have it set to appear). The default amount of lines to view at a time in it is 100 lines, yet never narrows it down to that few. And no matter what you change it to, it seems like it'll always be "All" in actuality. I don't know if the Minecraft launcher itself is related to the output window, but the launcher version is 2.0.1003 I believe (looking at the bottom left corner of it).

Edit: Looks like this extra paragraph already has its own report, MCL-5540 (which also made me realize there's a launcher issues section of this site, but I don't see a way to move this over to there... whoops)
On a side note, it goes and stays completely blank if it fills up with too many messages (starting from around 50k+ lines I think, and everything within the window frame disappears; nothing to do but close it at that point). Not sure if it would still freeze up like that with fewer visible lines, since that's currently impossible to test. Also, when it isn't broken yet but has a great many lines, the window starts to lag like heck when trying to interact with it (as if the framerate for it dropped to like 0.5 - 3 fps).

This is a bit off-topic, but it's also kind of annoying how you need to manually scroll down the output to catch up to whatever's going on; being able to optionally change that to something similar to how Twitch's chat boxes work would be nice...