[Solved]-Toggle button visibility on iframe fullscreen with highcharts-Highcharts

As a solution you can add a button as a child of a chart container, for example:

    chart: {
        ...,
        events: {
            render: function() {
                var chart = this,
                    x = 10,
                    y = chart.plotTop + chart.plotHeight + 50;

                if (!chart.customToggleBtn) {
                    chart.customToggleBtn = chart.renderer.button(
                        'Toggle fullscreen',
                        null,
                        null,
                        function() {
                            chart.fullscreen.toggle();
                        }).add();
                }

                chart.customToggleBtn.attr({
                    x: x,
                    y: y
                });
            }
        }
    }

Live demo: https://jsfiddle.net/BlackLabel/fbcmg392/

API Reference: https://api.highcharts.com/class-reference/Highcharts.SVGRenderer#button