Wildly OT: pop-up virtual keyboard for Mac or Linux?

[Skip Montanaro]

I know this is way off-topic for this group, but I figured if anyone
in the online virtual communities I participate in would know the
answer, the Pythonistas would... Google has so far not been my friend
in this realm.

One of the things I really like about my Skype keyboard (and likely
other "soft" keyboards on Android) is that when you hold down a "key"
for a brief moment, a little mini keyboard pops up, from which you can
easily choose various accented variants and other symbols. For
instance, If I press and hold the "d" key, I see these choices (ignore
the capitalization of the first letter - my mistake sending a text
message to myself from my phone, and I can't seem to convert it to
lower case):   Đ|¦&dðď

While I'm a touch typist, I almost never use auto-repeat, which is the
"binding" of held keys in most environments (curse you, IBM and your
Selectric!). These days I find my self needing accented characters
much more frequently than key repeat (C-u 2 5 - suffices in Emacs to
bat out 25 hyphens). Being an American with an American keyboard, I
haven't the slightest idea how to type any accented characters or
common symbols using the many modifier keys on my keyboard, and no key
caps display what the various options are. And I'm getting kind of
tired of going to Google and searching for "degree symbol". :-/

Is there an X11 or Mac extension/program/app/magic thing which I can
install in either environment to get this kind of functionality? I'm
thinking that if you hold down a key for the auto-repeat interval,
instead of the key repeat thing making all sorts of duplicates, a
little window would pop up over/near the insertion point, which I can
navigate with the arrow keys, then hit RET to accept or ESC (or
similar) to cancel. It need not be perfect. It might (for example)
only work in certain environments (Chrome, Emacs, vim, Firefox).
Anyplace to start. It need even be written in Python (though that
would be cool.) I think that once something like this caught hold, it
would fairly quickly take over from the dark lords of auto-repeat.

Thx,

Skip