the background position is bottom-left, therefor keyboard will push it up. consider a different positioning of the background image in order for the keyboard not to interfere. you can also look at other css properties that will meet your required output, overflow, background-size, position center fixed? For these specific Styling questions, the stack overflow forum is your best friend, way bigger community there to answer all the styling questions. Mendix forum is good for mendix tooling specific questions.
Maybe this one answers your question?