I did try to find this but did not see it.

IDE 1.8.5 is installed as the main IDE, RC2 is installed as a zip. Using Windows 10 home, fully patched.

1. I left the IDE open last night with a sketch for an Uno Wifi Rev2; this sketch was not saved.
2. This morning I copied a (non Uno Wifi Rev2) sketch from the forum and pasted it over the existing sketch; the new sketch is 242 lines and contains a error.
3. I pressed <ctrl>T; this might not be relevant.
4. I noticed that the status bar showed "building sketch".
5. More than 30 minutes later, the message was still there; I noticed one message about indexing but that disappeared again.
6. Closed the IDE and restarted it; empty sketch (no problem for me)
7. Pasted the forum sketch again
8. "Building sketch" shows again in the status bar, this time it does not hang and the message inthe status bar disappears.

I would expect the build process of the sketch to complete.

This might just have been a glitch; reported as an observation.