[Oberon] SystemV - Viewer Scroll

[Andreas Pirklbauer]

   > The exercise is still worth doing, question is whether backward scroll
   > with its double pass will yield noticeably faster update in the end. 

It is only worth doing if one *also* intends to implement *continuous*
line scrolling at the same time, i.e. where the contents of the text frame
is *continuously* updated when the user first presses the left mouse
key within the scroll bar *and* then holds it down *while* simultaneously
moving the mouse pointer up or down, i.e. just like in MS Word, see [1]).

In that case, the difference is rather striking - the new approach - which
uses CopyBlock whenever possible - completely eliminates flickering.

But if one does *not* want to implement continuous scrolling, i.e. if one
wants to essentially leave the FPGA Oberon as is, the original solution
chosen in FPGA Oberon (i.e. where the entire text frame is restored top
to bottom using TextFrames.Restore - even if one scrolls backward
only by single line) is amply sufficient.

-ap



[1] Continuous line scrolling demo video (click the “Download” button):

http://github.com/andreaspirklbauer/Oberon-experimental/blob/master/Documentation/DemoFractionalLineScrollVariableLineSpace.mov