Before 5.22, applets in largely contrasting themes (e.g. dark shell light apps) between shell and apps looked fine with maximised windows, mostly due to the transparency preventing "giant black rectangle in the corner of the screen." With the the panels now changing opacity on maximised windows, applets also become opaque when the panel is opaque, resulting in giant black rectangles in the corner of the screen again when maximised over white content, which looks ugly.
Hmm, I don't really understand this criticism. Before 5.22, the level of transparency was lower, so the appearance of dark popups in the corner of light apps was similar to what it is today when they are fully opaque. The actual change in appearance between the two would be quite small. If you don't like the contrast between a dark popup and a light app, don't use a hybrid dark/light theme.