The bug has been opened on https://bugs.launchpad.net/bugs/197110

"Binary package hint: gedit

If the last line in the viewable page is full of text but not wrapped, when enter is pressed to create a new line a scroll bar will appear reducing the viewable page size by the width of the scroll bar and causing the full line of text to wrap. When this happens, the wrapping pushes the newline down below the viewable area and it appears that the newline was not created. gedit will automatically scroll down to that line when any character is entered or the cursor is moved up and then back down, but it is not visible initially and the cursor is hidden on that line, so the user doesn't know where their text will appear when it is typed.

Reproduction steps:
1) open gedit.
2) put one character on a line and press enter as many times as you can without causing the scroll bar to appear.
3) Fill the last line with sequences of a few characters and spaces without causing the line to wrap or a scroll bar to appear.
4) Press enter at the end of this line. The line in step 3 should wrap due to the appearance of the vertical scroll bar reducing the width of the screen, and the newline created in this step should not be visible."