I tried again; things don't work out as expected.
Test input:
áà-äâ-ȧā-ạα
(two characters per layer: base: áà ; Shift: äâ ; AltGr: ȧā ; AltGr-Shift: ạα )
Stand-alone TCC:
View attachment 6200
TCC (.83), inside TCMD (.80), Start in ConPTY:
View attachment 6201
TCC (.83), inside TCMD (.80), Start in a New Tab:
View attachment 6202
The square blocks are a "I don't have this glyph in my font" non-issue. Copying and pasting the output lines to BabelPad confirms the overdot and underdot characters are read and passed around correctly.
View attachment 6204
A "dead key" is any character assignment that waits for the next character to be input and then outputs something; this is used typically, but not exclusively, for diacritical signs: (acute accent dead key) + a ⇒ á ; (diaeresis dead key) + e ⇒ é ; (Greek letters dead key) + a ⇒ α ; etc.
Dead keys, just like regular keys, may be on any of the four layers ("levels", in the parlance of the hideous ISO 9995 standard) a user types text with: base ("unshifted"), Shift, AltGr, AltGr-Shift, each accessed with, respectively: no modifier keys, Shift, AltGr and AltGr+Shift.
The AltGr key is the right-side Alt ["RALT"] key (maddeningly, Windows
still supports pressing LALT+LCTRL to get the same effect).
While the "plain" English (en-US) layout doesn't have any, and the most well-known international layouts only have a handful, almost always in the base and Shift layers, dead keys MAY be in any of the four layers. Some layouts use a lot: Spanish, Dutch, Swiss and several other western European layouts use five (for es-ES: acute and grave accents in the base layer, diaeresis and circumflex in the Shift layer, tilde in the AltGr layer), central European layouts have a full row in the AltGr layer, Finnish Multilingual has
19 dead keys in all four layers, etcetera. See
A Visual Comparison of Different National Layouts on a Computer Keyboard. for a rather comprehensive description of "international" keyboard layouts, writen by a certain stroppy git.