Scrolling not possible inside large PRE blocks

[vanife]

When the page contains a PRE tag with large (long and wide) text but limited in the area, the scrollbars appear.
It would be fantastic if one could enter/activate the area so that the usual scrolling would work inside the PRE area.

Entering this mode could be done:

• either similarly to the F for hyperlinks
• or by using functionality similar to the gf for cycling through the frames
• ... and exiting the mode with ESC

[int3]

In terms of implementation, this is essentially the same issue as #425, so I'm closing this.