Where is the arrange components inside area button in Altium 19?
There was an icon for that? I never used it - I try to memorize the 2- or 3-letter shortcuts, which is even faster than clicking an icon because you don't have to move the mouse away from the PCB and the second hand is already on the keyboard. 

For arranging components within a rectangle that shortcut is T-O-L. (Easy to remember, because it is a "great" feature, and "great" is "toll" in German :-P )
This shortcut didn't change in the past years.