Current viewport is reset when disassembling finishes

[jverkoey]

The viewport gets reset to bank 0 at the top rather than staying fixed to the current view.

A related bug is that if the bank being debugged is not open when a breakpoint is hit, then the bank's source will be opened but the correct location won't be jumped to until at least one more emulation step occurs. There is likely some race condition happening between loading the bank's source and the ability to jump to a given line in the source.