In "vi-editor" mode, the cursor can only positioned to a place after an actual character. If you click to the right of the last character in a line, the cursor will be placed right after the last character in that line. If you click below the last line, it will be positioned at the end of the text.
It is interesting to note that non-programmers usually find the "rand-editor" behavior more natural, whereas most programmers are used to the "unnatural" vi/emacs behavior.
You can change this setting in the launcher's settings dialog, under "Tools" - "Editor" - "ST80 cursor behavior at end of line".
(*) the "rand-editor" was a much advanced wysiwyg editor for glass-tube displays. It was quite popular in the 80's. (http://www.rand.org/pubs/notes/2006/N2239-1.pdf)