According to Adobe, this is caused by a bug in MacOS 10.11 that was fixed in MacOS 10.12.4. Adobe has tried to work around the problem in LR 6.10 / 2015.10 and later but has not been completely successful.
I scanned the bug report in the official Adobe feedback forum, and it appears that the problem mostly continues to happen only for those who haven't been able to upgrade to MacOS 10.12.4 or later. (Some of the posts in the bug report for later versions appear to concern different bugs.)
Moving forward, if this is still happening to you, please add details of your symptoms and the exact versions of LR and MacOS to the bug report (do Help > System Info). Be sure to click Me Too in the upper-right corner of the bug report. You'll be notified of any changes in status by Adobe. There's not much point in continuing the discussion in this thread, since Adobe product developers don't participate in this forum but do in the official feedback forum (containing the bug report).
Also, see this Help article for a workaround that's less inconvenient than restarting LR: Lightroom UI panels appearing black on macOS