[ARCH] Impossible for extensions to override Tab key behavior where code hints might be open

[peterflynn]

1. Install the Emmet extension
2. In a CSS selector, type "b" and press Tab
3. Clear the line. Type "b" again, but press Escape to close code hints before pressing Tab

Result:
When code hints are open, Tab selects the top item, inserting backface-visibility:|.
When code hints not open, Tab does the Emmet behavior, inserting bottom: |;.

Expected:
We should allow extensions to override key handling in the editor area even when code hints are (potentially) open, since code hints appear almost all the time when typing.

I have a similar case for JS snippets. E.g. I'd like to be able to type "clog"+Tab and get console.log("|"); inserted. But because JS code hints always come up with a camelcase suggestion of localStorage, this is blocked by code hints 100% of the time.

This used to work, but I think it broke in Sprint 26 when fba13c2 switched code hints to a "global keydown hook" which, once installed at the time the hints open, is effectively impossible to override. I looked into many ways of getting around this hook, but afaict it is 100% impossible. (There's not even an easy way to install your own higher-precedence global hook, since there's no notification to tell you when code hints have been opened).

[peterflynn]

CC @njx since he wrote the "global keydown hook" code in question, and @RaymondLim since I know he thinks about keyboard stuff a lot.

[JeffryBooher]

Moving this to Backlog to consider with API enhancements card.
https://trello.com/c/rnN0XwK0/876-3-research-extension-api-design

[peterflynn]

This is definitely out of scope for that story (we decided in Monday's arch meeting that no specific APIs would be considered -- only broad strokes). Reopening but leaving tagged MTB so we can find this a more suitable home on the backlog.

[njx]

Tagging [ARCH] - it's not totally obvious to me right now what the right fix is and it's probably worth discussing it at the architecture meeting.

[TomMalbran]

What about having priorities on the global keydown hooks? Emmet could then use a keydown hook with a higher priority than the keydown hook used by the Code Hints. Also, the keydown hook should receive a translated key to make it easier to distinguish between "Ctrl/Shift/Alt.. + key" from just "key". It can also receive the event, if is still required.

[peterflynn]

#4904 would render this whole issue moot

[TomMalbran]

The same issue could eventually appear with other keys.

[peterflynn]

Fair point -- and #4963 might eventually allow users to re-enable Tab handling for code hints, which would make even the Tab issue come back for some users. So, leaving open for now.

[pthiess]

Added a card in Trello, going to close this in here.