Edit: This is bug IDEA-256001, still affecting IDEA Community Edition 2020.3.
Using IDEA 2020.1, configured to use an installed JDK.
Using any of the ways of displaying Javadoc for a standard JRE class properly shows all the formatted text, but only has the "broken image" placeholder instead of the included graphics. Mousing over the placeholder shows the alt text, so it is parsing out correctly. This remains true when clicking the new "render in the editor" gutter icon, so that's orthogonal to the problem.
As an example, here's the editor window for javax.swing.text.StyleConstants in which is mouse is currently over the placeholder:
Things I've checked: Platform Settings > SDKs > Sourcepath, contains a long list of path\to\JDK\lib\src.zip!\some.module.name" entries, preconfigured by IDEA when I chose the JDK. (And the raw docs for all the standard JRE classes do show up correctly.) There is also a Documentation Paths tab on that window that's empty by default, but nothing I try adding under there made any difference.
I've found a couple other SO complaints about IDEA's javadoc support: this person didn't have the JDK's src.zip but mine does. Another person couldn't get their own images showing in their own docs and apparently IDEA couldn't do that at all in 2014. (The plugin offered as a solution for that question didn't work after Java 6.) I'm not trying to get anything from my own code to appear; I'm trying to view static documentation distributed along with the JDK.
What might I be missing here? I see in the example source's javadoc that the image is relative to a doc-files subdirectory, but I've not sure where that's supposed to be found. I'm expecting that the "real" solution is going to involve trawling around Oracle's godawful hostile website for zips...
