ClearScrollback should (have an option to) clear scrollback above active viewport

[eugenesvk]

Is your feature request related to a problem? Please describe.
Sometimes you scroll up to the output of a useful command but realize there is plenty of useless info even before that, which you'd like to delete to make scrolling more useful (it would stop at the start of that useful command output)

There is the default ClearScrollback command, which

Clears only the scrollback and leaves the viewport intact

But it doesn't leave the viewport intact, it clears everything above the bottom-most viewport even if your actual viewport is scrolled up

Describe the solution you'd like

ClearScrollback command should allow having an option to clear only the scrollback above currently visible viewport

Describe alternatives you've considered
Copy everything to a text editor and scroll/edit there

Additional context
N/a