Improper scrolling after drag & drop

[ProgerXP]

In Notepad2, dropping text near page start or end minimally scrolls the document to make the dropped text fully visible. In Notepad 2e, even with #41 ScrollYCaretPolicy of 0 but especially with >0 and on long lines with Word Wrap, the dropped text is usually made invisible. Because the 0 mode is supposed to behave like Notepad2, apparently this issue is caused by some of our additions, perhaps by #294.

For example, select last visible line by clicking on the gutter, then drag & drop it onto first visible line - document will be scrolled several lines down, hiding the dropped line.

Looks alike to #331.