My web forms center correctly in all browsers except for IE. They load correctly and center, but on a postback don't center. What is the very last event when loading a web page with IE

My web forms center perfectly browsers except for IE.  They center upon load, but with a postback, they don't center. It stays centered all the way through document.ready, pageLoad, the master page load and content page load, but there is an event afterwards that must be happening because it then moves to the left margin...no centering.
What is the last event to load with a webpage using IE 11?