If you are talking about the green and red sections that I’ve labeled before, the red is modal related/PlayCanvas UI related. The green is PlayCanvas UI related.
The background element Starting Screen has it’s anchor at [0.5 0.5 0.5 0.5] which means that depending on the window size, it may not fit and leave areas uncovered.
You can set the anchor to [0 0 1 1] which will always stretch the image to fit but may make the image look a bit odd as it’s not scaled proportionately.
Or I have a bit of code that does a similar thing to CSS cover here: https://forum.playcanvas.com/t/responsive-ui-for-all-device-in-playcanvas/15502/6?u=yaustar