Add support for `.XCompose`

• VSCode Version: Insiders ece7aaee [snap]
• OS Version: Ubuntu 20.04 LTS

ETA: Also occurs in latest stable (specifically ff915844 [snap])

Steps to Reproduce:

1. echo include \"%L\" > .XCompose
2. echo <Multi_key> <t> <h> <e> <r> <e> <f> <o> <r> <e> : "∴" >> .XCompose
3. Launch visual studio code.
4. In the main text editor (new file), <compose>therefore produces þerefore, and in addition <compose> by itself is missing its indicator (image 1). In integrated terminal, <compose>the produces he, still missing the indicator.

Images:

Does this issue occur when all extensions are disabled?: Yes

Expected vs produced: ∴ vs information in 4

[cab404]

So, I’ve kinda started digging into this problem. It would seem that this Chromium changeset is related: https://chromium-review.googlesource.com/c/chromium/src/+/3207928/

Which I found while digging for related issues in Electron: https://github.com/electron/electron/issues/29345

[Starwort]

I’m typing my compose key, followed by the string ‘therefore’ into the editor and instead of typing the ‘therefore’ character (∴ - defined in my XCompose), it types þerefore (‘þ’ coming from <compose>th, which is shadowed by <compose>therefore everywhere else in the system). I am typing, and the wrong compose key sequence is chosen (and I’m not sure exactly why, as there’s no reason that VSCode should be using a different input method to e.g. Chrome). If there’s more information needed I would be more than happy to provide it