You could view it as a modern reinterpretation of the Compose key, and I actually like the idea of having that key/functionality back without needing to remember obscure alt+number combinations.
That's an interesting comparison...
First, the position of the Emoji key is the Menu key's old position. It is about the same as
the position as the Compose key on old Unix keyboards such as those from Sun. On Sun keyboards with USB, the Compose key has the same USB HID keyboard usage code as the Menu key (since the inception of USB), and users of the Compose key on Unix/Linux and Windows (with a third-party program) with PC keyboards often map the Menu key to Compose.
But with the current USB standard, you can't have a keyboard that has both a Compose and a Menu key by default: you would have to map one or both in software to a different key/s.
I hope that Microsoft has assigned a previously de-facto unused but
existing standard key code to the Emoji key instead of making it fully proprietary: then Unix users would map that to Compose, and it could become considered a separate Compose key by convention.
The classic Compose key can already produce
some emojis, at least on modern Unix systems: <Compose> : ) produces ☺ for instance. But the classic Compose key is limited in that it interprets almost only two-character sequences, with a few three-character sequences.
For longer sequences to be viable, there would need to be better feedback to the user, and a way to cancel (other than running to the end of an invalid sequence).
I'd think that an interactive on-screen panel could be an improvement to the Compose key.
I went and tested the panel on a Windows machine, using the older Windows + . (period) key combo to open it, which works on older keyboards.
Weirdly enough, it does not grab keyboard focus to itself. Instead it forwards the input to the application beneath. If the active application has an active text field, it works like an
input method for it, working on input
in front of the original cursor position. Apparently, some text fields keep the cursor put not showing where input goes while others move the cursor, showing no marker where the emoji sequence started.
This gets weird if you open the emoji dialog over an application but there isn't an active text field.
I think a better design would at least grab keyboard focus to itself, and not include the input sequence into the text the user is working on.
Like it does now, it should show the input sequence (so far) and use it as a search string, narrowing down the search results as you type, allowing you to go between alternatives with the arrow keys and select with Enter.
With the existing panel, you cancel by backspace'ing to the beginning. I think Esc should also cancel.
The current panel does not detect code sequences as such: it only works like a search engine. However, it does not show the names that it matched to unless you hover the mouse over the icons.
The Emoji Panel requires the user to press a glyph to select it, while the Compose key exits compose mode and produces the glyph as soon as it has a full valid sequence.
The Emoji Panel did also not recognise any common Compose character codes that I tested: there are no ligatures (such as Ć - which is in the Danish and Norwegian alphabets), and only very few graphical characters.
The Compose key had been introduced way back when we used only 8-bit character codes, when the code set was relatively small. It would be awesome if it could be extended to work as an alternative to the Character Map for access to the full Unicode key set, with Greek, mathematical symbols and other things, and not just be used for emojis that had been added to Unicode for use by those who think that ASCII art is too hard.
Edit:
I noticed that GTK+3, has an emoji selector. It works only in programs based on the GTK+ library, which is used by e.g. Unix/Linux desktop environments Mate, Gnome and Cinnamon, and only where supported: The text editors and text fields. It does not work in the terminal.
It is patterned after Windows selector, and enabled with Ctrl + . (period).
It has its own text field and closes with Esc. Unlike Windows' Emoji Picker, it supports only emojis and the "tabs" for different categories are bookmarks within one large page of emojis...