I suppose it comes down to the fact that sidebars can now inhabit the space usually reserved for title bars. It’s not a design decision against which I have any

I suppose it comes down to the fact that sidebars can now inhabit the space usually reserved for title bars. It’s not a design decision against which I have any opposition so long as draggable space remains (and there usually is; I complained to Apple during the Big Sur beta period and they made some spacing adjustments). But I agree, those icons coming and going can make using them something of a pain. It also makes less clear what apps actually have them. An option in System Preferences to make proxy icons show all the time would be lovely.