First I thought there was a function as in Notepad++ called "Enable scrolling beyond last line" (really nice if you could add that).

Maybe it is only me that experience it as weird?

Ok, I see. So the problem is that deleting the last line should scroll the view up, so that the cursor stays on the bottom of the editor, to match the scrollbar.

When you start you need to have text so the scrollbar will be at 1/4.

Thanks for your report.
Though I was not able to reproduce it.

1. I open a file in an editor (local file if it matters). The contents is small enough to fit the window. I press Ctrl+End to move the cursor to the very end.
   
2. I press Enter several times to add empty lines so that the contents does not fit anymore. The original contents scroll out of the view and the vertical scroll bar appears.
   
3. I press Back several times to remove the empty lines. The scrollbar changes with every deleted line as expected.
   
4. Once I delete enough lines for the whole contents to fix the window, all contents scroll to the view and the scrollbar disappears.
   

Screenshots of all four steps are attached.

What version of Windows are you using?

When using the Editor in WinSCP i noticed a weird scroll behavior.

If the line will get more than fits in the window the scrollbar moves, but when i delete lines again it won't move back until i click with the mouse pointer on the scrollbar.

WinSCP 5.20.4.RC and it is the same in 5.19.6.