She was absolutely on top of what was happening

 Before her passing, Elizabeth spent her days doing business as usual. As noted above, she met with the prime minister two days before her death, on Sept. 6. “She was very, very keen to reassure me that we’d be meeting again soon. … It was very important to her,” Truss later said of the meeting. “She was absolutely on top of what was happening. Although she was physically quite frail, she was absolutely mentally alert.”

on top of

phrase of top

1. 1.

   on the highest point or uppermost surface of.

   "a town perched on top of a hill"
2. 2.

   in command or control of.

   "he couldn't get on top of his work"