I describe this as Fish-like as it is referenced as such by zsh-autocomplete, and I would define it as follows:
The appearance of a (temporary) automatically inserted completion match, that is highlighted in greyscale. Only when accepting the match the cursor position would be affected and normal highlight groups would be applied.
I think this concept of "phantom text" would be a nice extension to the naitive completion options. It would be a means for (future multi-line) completions to be inserted when selected from the menu, without moving the cursor or changing the view state of the window, depending on other completion options. I think this is doable with the asynchronicity these days.
Do you know if there have been efforts made to do this? Do you this is useful?
:keepjumps,:noautocmdand:doautocmdcould probably be used here. But with multiline completions in the making, the insertion should not affect scrolling or folds. Additionally the undotree should not be affected. I'm not sure how this would affect completion plugins. In fact, I'm not sure what's desirable overall.