Fullscreen not working

Hi @Weerayut_Prapamontol,
It appears to be a bug with fullscreen extension + bootstrap, thank you for noticing it. I forwarded this to our team, and they will work on a fix, unfortunately, I can’t give you any ETA.
As a workaround solution, I can suggest you to increase/decrease the gantt’s parent’s height on the onExpand/onCollapse events, like in this code fragment:

gantt.attachEvent("onExpand", function () {
  gantt.$root.parentNode.style.height = "100vh";
});
gantt.attachEvent("onCollapse", function () {
  gantt.$root.parentNode.style.height = "";
}); 

Here is the snippet:
http://snippet.dhtmlx.com/a20200660

Hi @Weerayut_Prapamontol,
This bug with the Fullscreen extension was fixed in the version 6.3.1: https://docs.dhtmlx.com/gantt/whatsnew.html#631
For now, the fullscreen extension works with the “bootstrap”, correctly without any workarounds, here is a demo:
http://snippet.dhtmlx.com/0c5564831