For your popup input box, you need to have the column size absolute to avoid change in size when the keyboard shows up.
The column scroll needs also different settings in column height and width. Try setting them to “fit content” and “fill container” to check which one would work.
@muneer Thanks for your help!! The setting of absolute height for the custom alert worked very well!
And the for the scrollable column I have set the height as Fit contents and put that in a column with height fill container for the background colour.
Thanks @muneer