My recollection from working on Gecko macOS text selection many years ago is that desktop platforms have different expected behaviors around how keyboard text selection interacts with existing selections, especially mouse-based. In particular: if you select some text, then shift-left-arrow or shift-right-arrow, what happens? Does it matter if the text was selected from left to right or right to left with the mouse?

We should audit this behavior on each desktop platform in a native text field, then make sure the framework handling matches on each platform.