Instead of only having one sandbox location (center_x/y/z+radius), allow it to be changed dynamically on-demand to anywhere on the server, for a given web client. Something like /websandbox tpahere user. Could be useful especially in conjunction with or similar to #47 spectator mode.
Requested in https://www.spigotmc.org/conversations/please-help-me-with-websandboxmc.211573/page-2#message-1263964