Hello,
Back in may I opened a thread now closed about “Theme not loading properly” but I am now happy to report that the last update of Webmin to 1.981 seems to have corrected all the problems/feelings I had back then. Interface is now perfectly responsive (I updated 4 servers, Ubuntu 20, Debian 10 and even one Debian 9) and left and right panes are loaded no problem. At first all servers were looping on the initial run script but a good cache clearing solved it right away 
We usually open threads when things are not right, well for once I authorized myself to open one to thank al the developers for the good work !
Pierre