I have seen similar issues have been addressed on the forum for AEM 6.1 and 6.2. The resolution was either eliminating relative sizes from the custom CSS or installing a hotfix. I seem to have a similar issue in AEM 6.4.
I can't see any CSS clash. Is there an existing known issue for AEM 6.4 related to this? My page contains an angular app. After the initial content load in edit mode the height on the ContentFrame iframe and ContentWrapper div increases to infinity.
Any help would be very much appreciated!