When selecting with the mouse and the mouse isn’t on top of a barline or a slur these can still be selected, which is quite annoying when you’re trying to select something . If it’s a barline you need to reselect and if it’s a slur it get’s moved so you need to undo. It’s as if there’s some kind of grid that the pointer attaches to, and you need to be at quite some distance from what you don’t want to be selected to make this work. This makes it hard sometimes to select what you want. Zooming in helps but it shouldn’t be necessary. It would be much better if this occurred only when you’re actually on top of the item (barline, slur etc).
I’m not completely clear what you’re describing here. Are you trying to make a marquee selection and finding that you’re starting the drag sufficiently close to an item that you end up selecting the item, rather than beginning the marquee, or something else?
Any bit of blank staff in the bar is typically a bigger target - easier to click - than trying to marquee select. Cmd/ctrl-click a blank bit of bar in another staff if you want a duet rather than a solo.
This is not to say that there isn’t possibly something that could be done to make marquee selections more accurate, but meanwhile you may have overlooked the easy option (particularly if it didn’t exist in the previous program you used).