There seems to be a bug in the web client where moving the cursor using the arrow keys are blocked if a checkbox is used.
Replication:
click the checkbox icon in the wysiwyg editor
enter some text
hit return, and a new checkbox will be automatically created
press the left arrow key and the cursor will move to the left of the new checkbox
press the right arrow and the cursor will fail to move to the right of the checkbox as expected.
If you move the cursor to the left of the new checkbox and put a space, this will allow the cursor to be moved passed the checkbox using the right arrow button, but will now prevent a checkbox from being automatically created on the next new line.