Add slur doesn't work (nicely) when range spans multiple staves

On a piano grand staff, if you select a range that includes notes from both staves such that there's only one note in each staff, you can't add a slur to them - only if you "list"/distinct select them (and even then, the default slurs added often aren't great, but can be tweaked as needed). Can't see any reason it shouldn't work with a range selection (exactly the same notes are selected!).
However if there are multiple notes in either staff, it adds a slur just to the notes in that staff, rather than one slur over the whole group of notes.
There are definitely cases it's useful to have separate slurs between the two staves, so I don't know what the exact behaviour should be here - as long as there's an easy and smooth way to add slurs that cross staves it's fine. Yes I know it can be done by initially entering the notes in one staff and "moving" them to the other one (such that they still belong to the initial staff), however that causes rests to disappear which isn't always what's wanted.
Example excerpts: https://www.ucolick.org/~sla/abcmusic/piano/cs2p2l1b1.gif
https://i.stack.imgur.com/Nfd2Q.png

Update: after playing around a bit more, the two main issues for me are:
a) that when the range selection has just two notes in different staves, it's not obvious why "add slur" does nothing. Perhaps some sort of status message to explain why nothing was done ("no staff contained multiple notes to add slur across") is sufficient?
b) the slurs that DO get created when you switch range to list selection (using the "Notes" button in the Inspector) are generally not very nice, and in some cases require a lot of adjustment. In principle the slur creation algorithm could just work the same regardless of whether the notes "belonged" to different staves or not, however I recognise there are some specific challenges here, and the "anchoring" won't necessarily be correct (meaning it might look OK initially, but not later when spacing changes or after copy/pasting etc.)