BuildCraft version: buildcraft-all-7.99.22
Forge version: 14.23.5.2815
Link to crash report or log: https://gist.github.com/RaiRaiTheRaichu/51e3f67f8b68a3014aefcbbf218c4846
Singleplayer or multiplayer: N/A
Steps to reproduce: Run the 1.12.2 instance from MultiMC, drag both the game window and the console window that pops up to a secondary monitor.
Additional information: Windows are being dragged from a 1440p monitor to a 2160p monitor, the loading will hang at that spot whether or not the game window is active the entire time or minimized.
Only seems to happen in my mod pack, not when the mod is isolated in an instance of its own, and it doesn't appear to be any specific mod in particular. I'm assuming it's not hanging, just slowing down substantially, which seems to worsen the more mods are added to the pack.

Looks like a mod causing issues with JEI (or maybe multimc), i'm not sure it's even buildcraft related. Can you reproduce this without buildcraft installed in that same pack? I'm also not clear if you're saying you can consistently reproduce this as-is WITH the "post-initializing guidemanager" or if that was a one time thing. What version of multimc are you on, and if it's not the latest dev/beta can you try that one (0.6.5-develop-1301 as of this post)? And more ram if possible (512-1024mb extra)?
The last noted mods in the log are binnies and immersive engineering... buildcraft seemed to load fine.
From the looks of it, it would load eventually, though obviously not desired behavior.

You should try out this jvm arguments in multimc:

-d64 -server -XX:+AggressiveOpts -XX:ParallelGCThreads=3 -XX:+UseConcMarkSweepGC -XX:+UnlockExperimentalVMOptions -Dsun.rmi.dgc.server.gcInterval=2147483646 -XX:+ExplicitGCInvokesConcurrent -XX:MaxGCPauseMillis=10 -XX:GCPauseIntervalMillis=50 -XX:+UseFastAccessorMethods -XX:+OptimizeStringConcat -XX:NewSize=84m -XX:+UseAdaptiveGCBoundary -XX:NewRatio=3 -Dfml.readTimeout=900 -Ddeployment.trace=true -Ddeployment.log=true -Ddeployment.trace.level=all