Screen navigation doesn't work at top or bottom

[arthrarnld]

Describe the bug
Screen navigation (ctrl+u, ctrl+d, ctrl+f, ctrl+b) doesn't work when the editor view is at the top or at the bottom of the file.

To Reproduce

1. Open a file.
2. Make sure to scroll all the way up. The cursor may be at any visible position.
3. Try pressing ctrl+d to move down. Nothing happens. Same goes for ctrl+u (in case the cursor is not already at the very top).

The same happens when you're at the bottom of the file. If the cursor is mid-line, any of the four shortcuts will cause it to move to the beginning of the line.

Expected behavior

Expected the cursor to move half a screen down (ctrl+d) or up (ctrl+u), or an entire screen down (ctrl+f) or up (ctrl+b).

Environment (please complete the following information):

• Extension (VsCodeVim) version: 1.11.2
• VSCode version: 1.39.2
• OS: macOS Catalina 10.15 (19A602)

[J-Fields]

Duplicate of #4179