I posted this problem before, but didn't get much response and now can't find that thread.
To recap this problem: when working in LR, the left side of the screen/window will become unresponsive. We can't seem to find a reason why this happens or some sort of cause where this problem occurs. This has been a problem since at least LR 5 (maybe 4) and has been occurring since at least OS X 10.8. Right now, we are using 10.10.5 and are using LR CC 2015.3. The only way to fix this problem is to quit Lightroom. This also occurs when we are working from the computer or from an external drive. We use individual catalogs per client job, and this occurs on multiple catalogs. I made a screen capture video illustrating this problem and some of the quirks with this bug. The link is below. I hope this helps you guys and gals find this problem as it can be quite annoying.
As you say this is a LONG term bug in Lightroom that has been around of at least 3 or 4 versions. When you get the double arrow you can click that point and usually drag the responsive/unresponsive areas back and forth. It seem like there is some hidden/invisible dialog box open. I have had this happen a few times and restarting Lightroom always has solved the problem but it is very irritating. I'm sure if someone could come up with a repeatable scenario that the Adobe engineers could fix it but it is an extremely random and luckily for me a rare problem. I know this does not solve your problem but this has been brought up on the forum several times with NO solution.