There seems to be a regression issue which was possibly introduced in Webix 5.4. If you open the following snippet
you will see a datatable in a popup window. The datatable has some rows populated but there is an empty one at the bottom. If you double click on any cell of the empty row to edit it you will see that the editor opens below the selected row - instead of in the same row. Then the datatable is pretty much messed up and when you click on a row to edit it, it actually edits the previous one.
This issue doesn't seem to exist in Webix 5.3 and the previous version.