Upward text selection stops at the top margin of text area

[hellojianqun]

Please provide the following information

Madedit-Mod version (or branch):
v0.4.14

platform/architecture:
Windows x64

please describe what symptom you see, what you would expect to see instead and how to reproduce it.
当按住鼠标左键向下选择长文本时，到达文档区域底部滚动条会自动向下滚动，从而可以自动续选；但是当向上选择长文本时，到达文档区域顶部就停住了，滚动条不会自动向上滚动。如果向上选择也能自动滚动的话，会方便很多呢。

[LiMinggang]

是我加了限制，可以放开