Don’t think you are doing anything wrong, this option seems to be broken for some time now.
Would be helpful to raise a support ticket for this with Mendix support.
Incase anyone finds this looking for an answer, here is some CSS I made to get the feature working for me - note the width in the CSS must match the width set for the sidebar
.mx-scrollcontainer.mx-scrollcontainer-horizontal.mx-scrollcontainer-slide {
position: relative;
display: flex;
align-items: stretch;
}
.mx-scrollcontainer.mx-scrollcontainer-slide .mx-scrollcontainer-center {
flex: 1 1 auto;
width: 100%;
margin-right: 0 !important;
}
.mx-scrollcontainer.mx-scrollcontainer-slide .mx-scrollcontainer-right {
position: absolute;
top: 0;
right: 0;
height: 100%;
width: 500px; //Side bar width
transform: translateX(100%);
transition: transform .25s ease;
z-index: 1051;
}
.mx-scrollcontainer.mx-scrollcontainer-slide.mx-scrollcontainer-open .mx-scrollcontainer-right {
transform: translateX(0);
}
.mx-scrollcontainer.mx-scrollcontainer-slide.mx-scrollcontainer-open .mx-scrollcontainer-center {
margin-right: 0 !important;
}
.mx-scrollcontainer.mx-scrollcontainer-slide.mx-scrollcontainer-open
.mx-scrollcontainer-right[style*="width: 0"] {
width: 500px !important; //Side bar width
}
I'd expect this to be an issue with the z-index being too low. So, for anyone still having this issue hopefully that might lead to a solution.