Has anyone else seen this problem? I am guessing this is more of an OS X problem vs. MacBook problem. Only when I use an external Dell 4K 27 inch monitor, not when using the laptop without a monitor, I frequently get areas of the window that won't respond to mouse left button clicks. For example the tabs in Safari or Chrome sometimes won't respond and switch tabs when clicked on. However, if I move the windows under it, they start working again. It seems related to what windows are stacked on top of each other. I will get some tabs to work, but not others until windows are shifted around. It is not only related to tabs but that happens most frequently. My current setup: Macbook Pro 13" 2015. Up to date with El Capitan and this has happened for a while with previous updates. Might be some weird thing with integrated graphics chip and OS X going on at the high resolutions. Again it never happens when just using the laptop with a mouse and no external monitor. I also have used the laptop with an iMac 27" 2009 as my external display and it worked flawlessly, just don't like the resolution it provides after seeing it in 4k.