The "new mouse select" is a feature (from what I can tell) that allows you to select an addition object on the "click release". Previously to select multiple object you held the shift key while clicking each object or stretch the select box over target area. Now it seems that in addition to this you are now able to select a second object, or transfer your current selected object when your release your finger from the mouse (AKA: click release).
My big problem with this is when you drag, rotate, or stretch an item you end up selecting a second one on the off click, or if you aren't holding shift it simply transfers your selected object to whatever prim your mouse happened to be over on the click release. This becomes extremely frustrating because from there you have a few very unwanted options; A) deselect whatever you targeted, B) re-select the objects you were targeting previously, C) If you were unfortunate enough not to notice that you selected an additional item until after you moved them, tap ctrl+z and start over. These may sound pety or "so what" problems, but building often requires alot of 'fussing around' moving object a little to the left, a little to the right. If after everytime you try to drop a prim, get a good look at it and go back you end up having to reselect your desired object you end up loosing time and efficiency (and for some of us patience). Imagine typing and after every time you hold shift for a capital letter that letter prints double, and you have to backspace so you're only left with one.
For those old enough to remember there was a feature removed from SL over a year ago that gave you the option to simply click an object and drag it without having to use the arrows. Though I was, along with many others, aggitated at this decision I did understand it's practicality. Poeple were accidently relocating houses, stores, vendors, ect, every hour of the day.
With this new feature however, I fail to see any practical reason or convenience for selecting an object whenever you release your mouse. If I had missed a patch note, if there is a reason for this, if this is a bug, if it has been recognized by LL already, or if there is a way to turn it off. Please post a reply and I will be eternally greatful.
Thanks for your ear-
Fatal Raine
EDIT: I've been trying to duplicate this problem to better explain it and I'm finding that it seems to be very fickle. Sometimes it only occurs when using the "edit linked part" option of a linked object. Other times I'm selecting accidental object all over the place. The inconsistency is making it seem more and more like a bug.