It seems a long standing QT issue (4, 5, 6). It can be fixed by rebuilding from source, but the solution for recent Linux versions and desktops is below
Oddly some Compose key combos do work
The solution
In console make iBus default input method
im-config -n ibus
Edit the .XCompose in home, adding at start
include "/usr/share/X11/locale/en_US.UTF-8/Compose"
Enable show hidden and create if it doesn't exist
I use a UK Keyboard layout, but there is no en_<anything other than US> The /usr/share/X11/locale/en_US.UTF-8/Compose does seem to have all the default keys. Compose _a for ā Com oo for ° etc
All the common ones now like ° — – … ′ ″ work in Calibre, Viber and KATE (which I didn't realise only partly worked).
Obviously you have to Kill & restart Viber, or exit KATE, Calibre etc.
The AltGr keys are unaffected, eg þáŋçߧłŁ
My custom extra compose keys for Greek, prime and double prime:
Code:
include "/usr/share/X11/locale/en_US.UTF-8/Compose"
# GREEK CAPITAL LETTERS
<Multi_key> <G> <A> : "Α" U0391 # GREEK CAPITAL LETTER ALPHA
<Multi_key> <G> <B> : "Β" U0392 # GREEK CAPITAL LETTER BETA
<Multi_key> <G> <G> : "Γ" U0393 # GREEK CAPITAL LETTER GAMMA
<Multi_key> <G> <D> : "Δ" U0394 # GREEK CAPITAL LETTER DELTA
<Multi_key> <G> <E> : "Ε" U0395 # GREEK CAPITAL LETTER EPSILON
<Multi_key> <G> <Z> : "Ζ" U0396 # GREEK CAPITAL LETTER ZETA
<Multi_key> <G> <H> : "Η" U0397 # GREEK CAPITAL LETTER ETA
<Multi_key> <G> <I> : "Ι" U0399 # GREEK CAPITAL LETTER IOTA
<Multi_key> <G> <K> : "Κ" U039A # GREEK CAPITAL LETTER KAPPA
<Multi_key> <G> <L> : "Λ" U039B # GREEK CAPITAL LETTER LAMDA
<Multi_key> <G> <M> : "Μ" U039C # GREEK CAPITAL LETTER MU
<Multi_key> <G> <N> : "Ν" U039D # GREEK CAPITAL LETTER NU
<Multi_key> <G> <P> : "Π" U03A0 # GREEK CAPITAL LETTER PI
<Multi_key> <G> <R> : "Ρ" U03A1 # GREEK CAPITAL LETTER RHO
<Multi_key> <G> <S> : "Σ" U03A3 # GREEK CAPITAL LETTER SIGMA
<Multi_key> <G> <T> : "Τ" U03A4 # GREEK CAPITAL LETTER TAU
<Multi_key> <G> <U> : "Υ" U03A5 # GREEK CAPITAL LETTER UPSILON
<Multi_key> <G> <F> : "Φ" U03A6 # GREEK CAPITAL LETTER PHI
<Multi_key> <G> <X> : "Χ" U03A7 # GREEK CAPITAL LETTER CHI
<Multi_key> <G> <O> : "Ω" U03A9 # GREEK CAPITAL LETTER OMEGA
# DOUBLES (additional stroke <Q>)
<Multi_key> <G> <Q> <T> : "Θ" U0398 # GREEK CAPITAL LETTER THETA
<Multi_key> <G> <Q> <O> : "Ο" U039F # GREEK CAPITAL LETTER OMICRON
<Multi_key> <G> <Q> <X> : "Ξ" U039E # GREEK CAPITAL LETTER XI
<Multi_key> <G> <Q> <P> : "Ψ" U03A8 # GREEK CAPITAL LETTER PSI
# greek small letters
<Multi_key> <g> <a> : "α" U03B1 # GREEK SMALL LETTER ALPHA
<Multi_key> <g> <b> : "β" U03B2 # GREEK SMALL LETTER BETA
<Multi_key> <g> <g> : "γ" U03B3 # GREEK SMALL LETTER GAMMA
<Multi_key> <g> <d> : "δ" U03B4 # GREEK SMALL LETTER DELTA
<Multi_key> <g> <e> : "ε" U03B5 # GREEK SMALL LETTER EPSILON
<Multi_key> <g> <z> : "ζ" U03B6 # GREEK SMALL LETTER ZETA
<Multi_key> <g> <h> : "η" U03B7 # GREEK SMALL LETTER ETA
<Multi_key> <g> <i> : "ι" U03B9 # GREEK SMALL LETTER IOTA
<Multi_key> <g> <k> : "κ" U03BA # GREEK SMALL LETTER KAPPA
<Multi_key> <g> <l> : "λ" U03BB # GREEK SMALL LETTER LAMDA
<Multi_key> <g> <m> : "μ" U03BC # GREEK SMALL LETTER MU
<Multi_key> <g> <n> : "ν" U03BD # GREEK SMALL LETTER NU
<Multi_key> <g> <p> : "π" U03C0 # GREEK SMALL LETTER PI
<Multi_key> <g> <r> : "ρ" U03C1 # GREEK SMALL LETTER RHO
<Multi_key> <g> <s> : "σ" U03C3 # GREEK SMALL LETTER SIGMA
<Multi_key> <g> <t> : "τ" U03C4 # GREEK SMALL LETTER TAU
<Multi_key> <g> <u> : "υ" U03C5 # GREEK SMALL LETTER UPSILON
<Multi_key> <g> <f> : "φ" U03C6 # GREEK SMALL LETTER PHI
<Multi_key> <g> <x> : "χ" U03C7 # GREEK SMALL LETTER CHI
<Multi_key> <g> <o> : "ω" U03C9 # GREEK SMALL LETTER OMEGA
# doubles (additional stroke <q>)
<Multi_key> <g> <q> <t> : "θ" U03B8 # GREEK SMALL LETTER THETA
<Multi_key> <g> <q> <o> : "ο" U03BF # GREEK SMALL LETTER OMICRON
<Multi_key> <g> <q> <p> : "ψ" U03C8 # GREEK SMALL LETTER PSI
<Multi_key> <g> <q> <s> : "ς" U03C2 # GREEK SMALL LETTER FINAL SIGMA
<Multi_key> <g> <q> <x> : "ξ" U03BE # GREEK SMALL LETTER XI
# prime etc
<Multi_key> <0> <apostrophe> : "′" U2032 # prime
<Multi_key> <0> <quotedbl> : "″" U2033 # double prime
My .XCompose