The inline editors are actually different HTML elements that are displayed at the same position as the grid cells. But they are located at a different DOM position, so they don’t follow the grid cells. Moreover, when you resize the grid or collapse/expand tasks, the grid is repainted. It means that all HTML elements are recreated. In previous Gantt versions that would mean that the inline editor would be closed: http://snippet.dhtmlx.com/5/4b37aad7a
I will add it as a bug to our internal tracker. The dev team will fix it in the future, but I cannot give you any ETA.
Right now, to save the inline editor position, you need to manually implement a custom solution.
Here is an example of how it can be implemented: http://snippet.dhtmlx.com/5/c907bc9e3