I've been seeing the most bizarre behavior in Netbeans lately: when hovering over buttons to see the tooltip, the tooltips frequently (like 99% of the time!) show for a split second (not long enough to actually read them) and then disappear again! Sometimes the tooltip can be made to 'stay up' if one wiggles the mouse a bit.
This just started recently - and I have no idea what caused it. I haven't downloaded a new version of the JDK recently (I'm still on 1.8.0_144) and my OS (macOS 10.13) hasn't had any updates either, I think. There have been a couple updates to NB 8.2 that I accepted - so maybe it was one of those? The Swing application that I'm running within NB (using JDK 8 platform also) doesn't have this issue, sort of confirming it to be an NB-only problem.
I just downloaded, built, and ran NB 9 using JDK 9 for the first time today. And it has the same issue - tooltips just flick on and off!
I thought it might be a problem specific to the L&F - I use Darcula - but when I tried it with "MacOS" L&F, I see the same problem.
I did notice that the problem seems less severe (i.e. happens less often) when the checkbox "Maximize use of native look and feel" is not selected.
Has anyone else seen this? I don't see anything recent in the bug database when searching for "tooltips".