I checked that with the patch, the wwindow top stayed synchronized with the text after multiple movement operations of different types.

The minimum requirement for correct operation of the scrollbar slider is that moving the slider to the top and bottom of its trough moves the text to the top and bottom of the file.  Before merging, I checked with editor.py, about 3000 lines, and a new editor with just a couple more lines than the window size.

It is desireable that each pixel movement of the slider move the text.  This requires that there be at least as many hidden lines in the text as there are exposed pixel in the slider trough.  I don't know that this will always be true on all systems.   As near as I could tell with editor.py, which meets this requirement, moving the slider a pixel away from top or bottom also moved the text.  (I can't move a single pixel, so it was more a matter that moving 2 or 3 pixels made 2 or 3 jumps ;-)

I considered these tests enough for now to merge.