“The game does not “steal focus” in the browser, which means that the arrow keys and space button can not be used. You need to fix this and re-upload.” - I just got this from a website I tried to upload my game to. How to solve this issue?
I’ll look at it and notify you when I got a solution 
Try this:
Go the Game Develop folder, then in JSPlatform, Runtime and open runtimegame.js ( with Notepad or a good text editor like Notepad++ )
Search for the line where there is renderer.view.onmousedown = function(e){ and paste window.focus(); below, just like this:
renderer.view.onmousedown = function(e){
window.focus();
game.onMouseButtonPressed(e.button === 2 ? 1 : 0);
[...]
Then, locate the line beginning with window.addEventListener(‘click’, function(e) { ( a bit after ) and paste again window.focus(); the line after:
window.addEventListener('click', function(e) {
window.focus();
e.preventDefault();
return false;
}, false);
Save the file.
Then, export again your project and let me know if the website does or doesn’t complain anymore about stealing focus