For OneStepCheckout versions lower than 4.5.*

Modal windows in mobile devices are not positioned correctly when user tries to edit fields in modal (like logging in etc) and can not be used. This is related to a position calculation bug in livepipe.net modal window javascript library that OneStepCheckout is currently using.

To solve the issue open up skin/frontend/base/default/onestepcheckout/js/onestepcheckout.js and add the following configuration value to line 224:

position: 'center_once'

so it will look like this:

this.popup = new Control.Modal(this.popup_container, {
overlayOpacity: 0.65,
fade: true,
fadeDuration: 0.3,
position: 'center_once'
});

this results that the popup placement is calculated on creation time and will not be recalculated when viewport (window size) dimensions change. user can then swipe to focus the window and fill the forms.