It turned out my old workspace and my fresh one had unrelated problems, and I managed to fix my old workspace.

My old workspace and its project was originally set up to have a Java 1.7 "Compiler compliance level" (resp. Preferences -> General -> Java -> Compiler, and right click on project -> Properties -> Java Compiler), which isn't supported anymore. When I accessed these menus, Eclipse showed me a warning telling they changed it to 1.8, but obviously the change wasn't effective. I had to set it manually to 1.8 even though it appeared to be 1.8.

In my fresh workspace, Eclipse waits for me to edit a file before finishing highlighting it. I don't know why but I won't investigate because I'll keep using my old workspace, which is now OK.