There might be some css in your code which might be causing the above issue. Try checking any css files which you might overwrite from libs.
Easy way is to create new 6.4 instance without custom code and compare the styles using browser inspect element, so that we can find the css which is causing the issue. Also, check if there any js errors in console.
Also, try adding ?debugClientLibs=true param in the url, which loads individual css files and it will be easy to find path of the stylesheet causing issue.
I upgraded from AEM 6.2 to AEM 6.4 and did not face this issue.