See attached example. This uses MessageBox() from the Windows API.

Double click on a cell [dialog opens], place the mouse over a cell next to the one you clicked on, then press ESC. Alternatively, place the dialog's OK-button over a cell next to the clicked-on one and press OK.

The selection in the table will be extended to the position your mouse was at. After debugging through Qt it seems that the mouse press that is sent for the double-click event is still "active" after the native dialog is closed. The next mouse move event will then have a button pressed along with it, causing the selection to be extended.

This is a bug that was also true for Qt-Dialogs in 5.2.0, but was fixed in 5.3.0 (possibly through QTBUG-25831, but this is a guess).