Using mobile browser (Safari iOS or Chrome Android)
Go to demo grid page
Click any grid's filter menu which contains text field
Try to enter text into textbox.
The filter menu automatically closed, so you can't enter anything
this happens because the keyboard pops up, changes the screen size and may even scroll the page, and both of these events hide the popup menu.
A workaround could be using the filter row mode, with the caveat that it takes up more space on the initial rendering, and that filtering happens immediately.