The introduction of using the Alt key to allow making a selection window in Layout without unintentionally moving the content is a nice addition but I think it doesn’t quite go far enough. The current plus/minus functionality of using the shift key when making a selection window usually results in additional picking of individual items to either remove or add to the initial selection window.
For example, say that your initial right-left selection window included parts of multiple larger groups on the same page, in order to deselect those groups you now have to individually deselect each group which was not intended to be a part of the original selection window; this process can get complicated as the number of groups or objects on a page increases and making additional selection windows using the shift key adds or subtracts items which were not intended to be part of the selection.
Thus, just as in SU, I think it would be great if it were possible to have modifier keys to either purely add to, subtract from, or the current use of shift: add/subtract.