For personal historical reasons I’m used to US keyboards, so to write portuguese text with those keyboards I need to use the English (US, int. with dead keys) layout that provides compose sequences to write characters like áàãéíóóõúüç.

To generate a ' character I type ' +     . It’s this way since ever…. but lately (starting from GTK3 version 3.24.28) somebody decided that this was a bug, not a feature. If someone type this compose sequence the generated character should be ´, not ', and to generate ' you would need to type AltGr + '. The problem is that in protuguese the character ´ alone have no meaning/usage, on the other hand the character ' is used a lot when programming, i.e. this had a huge impact on me.

Long story short, to let GTK behave like I’used to, create a file named .XCompose in your home directory with this:

<dead_acute> <space>                : "'"   apostrophe # APOSTROPHE

If you are not used to the XCompose format, check /usr/share/X11/locale/en_US.UTF-8/Compose and you are going to have a idea (or man 5 Compose).

In the past the behavior change that made a headache was ' + c generating ć instead of ç as I was used to, the new compose sequence (that now I’m used to) is AltGr + ,. Hope it helps whoever is reading.