In the attached workbook, Dashboard1 uses a method of popping one sheet over another. (Yes, this could have been done with sheet swapping, but the point here is to demonstrate a different way of displaying what 10.5 is messing up.) This simulates the behavior I've been using all over the place since I started using Tableau in V 8.0. In desktop we cannot drill to the lower object even if the upper object has collapsed away. But when published the user was able to click on the lower sheet when the upper sheet was gone. It was a beautiful undocumented feature.
From another thread (https://community.tableau.com/thread/266084) I have been discussing this issue with Brian in email, and he found out that Development has made the server behavior match the Desktop behavior starting in 10.4. This is a sad day for me. It was always an undocumented feature, and a long-standing principle of programming and software use is that undocumented features are at risk of being considered a bug by the software vendor, and that day has arrived for Tableau.
I have a workaround.
Dashboard 1(2) places all the sheets in one strip of a long container. My popout sheet pushes everything to the right when the parameter is set to "Y", and collapses to the left when the parameter is "N". The Sales Forecast sheet is at the end of the line of objects in the container. That's what we see when the parameter is "N". The "What If" sheet gets pushed into the middle of the dashboard when the parameter is "Y", and Sales Forecast gets pushed to the right. I floated a container over the space where Sales Forecast lands when it is pushed to the right. This masks that sheet from view for the user, and "What If" remains displayed. (I colored the container white so to the user it just looks like empty space.) We can place other objects on top of this masking container.
I look at this technique as having a "stage" where different scenes of a play occur. Stuff to the right and to the left of the stage are hidden behind curtains of one sort or another. In this example, one curtain is the left border of the dashboard, and the right curtain is the white container.
I know already that I have some pretty complicated uses of the old behavior. Each can employ the principles of what I did in the attached sample, but some of the fixes won't be easy for me. Still, we can address it, and I won't let this new behavior beat me.