Fix issue with cursor at the end of wrapped line

[timdown]

Why

This fixes the same flashing cursor problem described in this comment in the Vim bindings repo. The Emacs bindings have the same problem and the same fix to the block cursor.

What changed

When a line wraps at a space, the cursor can no longer be placed after the space at the end of the first visual line.

Test plan

To see the bug:

1. In dev/index.html, change the editor width to 30ch
2. Run the dev server and navigate to it in a browser
3. On line 3 of the editor, place the cursor after the space that follows from at the end of the visual line
4. The cursor flashes and contains the next character, which is '. With this fix applied, the cursor cannot be placed in that location.

[nightwing]

Thank you!