Edit Feature with Cellnav not consistent on last row when hiding horizontal scrollbar

We detected this issue with @andresbiso and @zargoht

Plunker: http://plnkr.co/edit/n2Lf5A4yOBbzCXzkm6fp

It's a basic UI Grid with ui-grid-edit and ui-grid-cellNav features

<div ui-grid="gridOptions" ui-grid-edit ui-grid-cellNav class="grid"></div>

and the horizontal scrollbar hidden.

$scope.gridOptions.enableHorizontalScrollbar = uiGridConstants.scrollbars.NEVER;

Steps to reproduce

1. Scroll down to the last row of the grid
2. Single-click the Name cell of the last row ("Marci Emerson" on the example)
3. Press a character key

It should replace the cell's content with the character inputed as happens with the other rows. Instead of this, it opens the editing field with the previous value loaded and with the input cursor at the end. So pressing new characters will append them at the end of the existing text instead of replacing it.

Note: this behaviour isn't consistent. Sometimes it behaves as expected and sometimes the issue happens. You have to repeat the steps 2 and 3, eventually it will happen.