Hi, the bug comes with multi-languages websites and particular web-browsers.
My configuration:
joomla 3.9.1
T3 Framework 2.7.3
Browsers: Firefox 63.0.3 (64-bit), Chrome 70.0.3538.110 (64-bit), MS Edge (win10)
My website is English + Russian languages
Tested locally (Denwer, php 5.6.19, mysql 5.5.25, win10) and VPS hosting ( Plesk Onyx 17.8.11, mysql 5.6.42 , Apache, PHP 7.2.12, Ubuntu 14)
The problem is: megamenu is hidden when I open English version of website in Chrome and Edge. Otherwise everything is Ok.
Solution: \plugins\system\t3\includes\renderer\megamenurender.php, line: 59
Change
$langcode = JFactory::getDocument()->language;
To
$langcode = JFactory::getLanguage()->getTag();
Works Locally and with VPS.
Some description: As I see, JFactory::getDocument()->language; returns "ru-ru" for English version of website in Chrome and Edge.
Thank you very much for your great job!