Ensure Prometheus graph overlay is in front of cursor flag
What does this MR do?
Fixed a bug where the graph "cursor flag" blocks the hover position calculation for the graph behind it, so if you scroll too fast to the right, you'll often accidentally hover over the date/time text and the graph position will no longer update to follow your cursor until you move it off of the flag.
Before:
After:
What are the relevant issue numbers?
Closes #37102 (closed)