When the UpdateUI method is called, the selected cells are cleared. 

When passed a collection of cell regions and the SelectionMode property is Extended, the SelectCellRegion method selects only the last cell region.

The scenario:

We have a document with text inside and we are executing the following steps:

1. Mouse left button down.
2. Select part of the text.
3. Replace the selection with new text programmatically (the text should be with a different length).
4. Release the left button.

Observed: Before releasing the mouse button the cursor position is correct but after releasing it the cursor position is changed to the initial selection end.