When an element inside the modal dialog has position:fixed, the transforms affect the calculation of the element's position. This creates a problem when tinymce is viewed fullscreen.
Change transform: translate(0, 0); to the following in your style sheet:
.modal.in .modal-dialog {
-webkit-transform: none;
-ms-transform: none;
-o-transform: none;
transform: none;
}
Updated fiddle: jsfiddle.net/344y9brr/4/
The transform: translate(0, 0); creates a containing block for fixed-position descendants. See this question why -webkit-transform messes up with fixed childs for more info.