Suddenly I can’t select any assets on the Scene window by directly left-clicking on them. This seems to have started since the last Unity upgrade to 5.3.4f1. I can perform a selection if I do a click-and-drag marquee selection but not by clicking individual assets. Previous suggestions of closing and re-opening the Scene window didn’t fix it.
I was using the FX/Water (WaterProDayTime) shader from the Standard Assets and by disabling or switching it to a different shader this problem goes away.