set viewport

viewport.js

// Option A: reset maximum-scale on blur

var viewportMeta = $('meta[name="viewport"]');
$('select[name="filter"]').bind('focus blur', function(event) {
    viewportMeta.attr('content', 'width=device-width,initial-scale=1,maximum-scale=' + (event.type == 'blur' ? 10 : 1));
});

// Option B: limit maximum-scale to 1

var viewportMeta = $('meta[name="viewport"]');
$('select[name="filter"]').bind('focus blur', function(event) {
    viewportMeta.attr('content', 'width=device-width,initial-scale=1,maximum-scale=1');
});

// Option C: prevent focus entirely

var viewportMeta = $('meta[name="viewport"]');
$('select[name="filter"]').focus(function() {
    return false;
});