for me it meanwhile worked to use an html snippet with following statement: <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=0"/>
also looking for an answer to this question
Just a suggestion. You say this only happens at every inputbox. My first check would be to see if this also happens with a regular, non-Mendix input box.
Another option is to try input-addons that are available on the Marketplace: