Unfortunately due to bug reported here it’s impossible to scroll through long logs - they’re hidden.
Also, performance of the build page during build (i.e. appending the logs live) is very poor - it consumes the most resources of all pages currently opened in Chrome on my computer. The fans of my MBP 13" start to work hard few seconds after the build started. I can issue a bug related to this if it wasn’t reported earlier.
Here is a screenshot from Chrome task manager:
