When overal page box-sizing is set to border-box width of .ui-dialog-content is not correctly calculated. You can see that when starting dialog resize. Some additional empty space appears between vertical scrollbar and dialog right border. Setting this style:
box-sizing: initial !important;
somehow solves the problem. But I haven't tested yet what are the side consequences.
You can see an example at: https://jsfiddle.net/2xLy19y8/
Just try to resize dialog to small size and you will see...
Thanks for attention!