I note that this has been marked "Solved", yet it is still an issue (e.g. to anyone finding their new system install makes Screeps crash raises hand).
What has been found is merely a workaround, not a solution - especially if that workaround breaks other things (@ags131 notes that it affects Thunar - something that is perhaps more important to have working than keyboard input in Screeps!).
The latest version with which the Steam client works is 1.44.1, as @godman180 noticed. (1.44.1 has a date of 27-Jul-2019 in the arch repository.) I did check that no version later than 1.44.1 (up to the most recent one - 1.44.7 25-Oct-2019) works; I also tried using different versions of harfbuzz (as that sounded like it might be the specific cause, rather than pango itself), but the client won't start with any version below 2.6.0, and crashes on input with that version.
Please reopen this, and only mark it as solved when the client no longer crashes with the newer versions of pango. Even if this is an issue with proton (which something tells me it might be).