I just came across this little peculiarity -
when resizing the view, editorhost expects IPlugView::checkSizeConstraint () to return anything except kResultTrue if the PlugIn doesn’t approve the new size and wants another one (editorhost.cpp:375).
host-checker, however, happily ignores that concept and always returns kResultOk, even if it modifies the ViewRect (vst3editor.cpp:1489).
The documentation for IPlugView::checkSizeConstraint () supports both ways, since it simply doesn’t detail anything about what the return code might signify.
Vst3PluginTestHost obviously ignores the return code, since it respects host-checker’s returned ViewRect.
It might be a good idea to unify (and correctly document!) the behavior.