Stacking VST3 instances impossible?

I noticed that the only functional difference between VST2 and VST3 is the additional input knob in the mixer section of the VST3 version.
Is it that control that disables stacking several instances in a DAW rack? With the VST2 version that is possible.

Another minor difference is that with the VST2 version the DAW’s plugin window is not wide enough, which makes the vertical scroll bar hide the right wood panel . Seems that the plugin reports the wrong pixel width to the DAW.

Thinking about it, I just noticed that in the VST3 version the scrollbar is missing altogether. So I cannot see the bottom 1 cm or so of the virtual keyboard. On the other hand, the missing scroll bar makes the right wood panel display correctly.