Kibana has become ugly in vewing JSON documents since some version, I'm not sure exactly which one, let's consider these two: v7.7.0 (normal) and v7.15.1 (abnormal).
The problem is developers has replaced browsers' native selection of the text with emulated selection, therefore many functions of context menu doesn't work anymore. Even copying selected text got impossible. Moreover, the performance of rendering has dramatically decreased, browsers are literally hanging during rendering visualization of JSON.
So, I call on developers. Please, return back the output of JSON inside 'pre' tag, do not wrap it into complex HTML structures, do not prevent browsers' native behavior on working with text.