I have all updated OS 10.1.6 elCap and LR 2015.12 and I still randomly have the dark panels issue. iMac Late 2015 *GB ram and AMD Radeon R9 M380 2048 MB.
I know it is a know issue and was hoped to be resolved earlier this year. It is still happening intermittently but frequently. Any ideas for solutions? Yes, minimizing the window and reopening resolves it but it comes right back.
I have all updated OS 10.1.6 elCap and LR 2015.12
To correct this issue, Adobe recommends updating to Sierra 10.12.x (currently 10.12.6).
Workarounds are offered for OS X 10.11 El Capitan and earlier.
Can you update to Sierra? Or do you need to stay with El Capitan for some reason?