Cannot select&copy job log

Problem

You can see a job log in job details page. Recently, this page seems slow and sometimes unresponsive.

Here is an attempt to select some texts for copying the content. As you can see, light-blue selected visual doesn't appear even though I'm trying to hold a left-click-button of a mouse and drag.

I've been seeing this symptom for the last few weeks. Previously, this was working smoothly.