Dear @aleksandra_budnik,
Indeed your questions were very pertinent. I removed a number of overflow and display CSS settings at the different levels of the modal parts hierarchy and the situation is much better; the spreadsheet shows up when the modal appears. It shows up incompletely though, it requires an interaction ( a click anywhere on the spreadsheet ) to get it completely with the headers and dropdown menus. And even then labels and dropdown menus arrows overlap. As if the speadsheet does not “dare” to occupy the available space.
So you see, a real and substantial improvement, thank you !
And still some last progresses to make.
Let me try to write an excerpt of my ( quite complex ) application to use as a demo.