Lots of really great discussion and comments here about measuring scope bandwidth!
We get so many questions about this topic, I actually made a video explaining the basic concepts of "bandwidth" in oscilloscopes. Probably a bit too introductory for most of the people posting here, but it does cover a few things discussed in this thread such as calculating BW from rise time, the different frequency responses (Gaussian vs. flat), etc.
One thing I didn't cover is how to measure the bandwidth of an oscilloscope (i.e. procedures, techniques), but it seems like this might be a good topic for a future video. I've already learned a few things from this thread myself