Rich Text Editor breaks the /rainbow command

[ghost]

Enabling the Rich Text Editor (RTE) stops /rainbow autocompletion and rainbowfying messages.

Steps:

1. With RTE disabled, start typing /rainbow (autocompletes to show the explanation of the rainbow command)
2. Continue typing so your message reads /rainbow _message_ and press ENTER (message appears in the room coloured as a rainbow
3. Go to Labs and enable RTE
4. Start typing /rainbow followed by a message and press ENTER

Result:

• With RTE disabled the autocomplete explaining the rainbow command appears and any message that follows is formatted coloured as a rainbow.
• With RTE enabled the autocomplete explanation does not appear and what gets output is /rainbow message in plain, unformatted text

Tested this on Fedora Linux 37 with Element Desktop 1.11.16 installed via Flatpak from Flathub.org. I also replicated this on Element Android 1.5.10 on a Pixel 7 running GrapheneOS Tiramisu, Element iOS 1.9.13 on an iPhone 12 Mini running iOS 16.1.2 and staging.element.io on Firefox 107.0.1 on Fedora.

Here is a screenshot of the autocomplete with RTE disabled:

Here is a screenshot of the message formatting, the first grouping of messages is with RTE enabled, the last message is with me entering /rainbow message with RTE disabled:

I'd be happy to provide any other information that is useful.

[t3chguy]

Slash commands aren't functional at all in the RTE yet - see element-hq/element-web#23880

[alunturner]

The rust side of this implementation was added in v1.2.0. Corresponding react-sdk work carried out in PRs:

• Handle command completions in RTE matrix-react-sdk#10521
• Handle more completion types in rte autocomplete matrix-react-sdk#10560
• Commands for plain text editor matrix-react-sdk#10567

Opening a related issue to deal with the case where a user inserts a command inside a paragraph tag in the rich text editor. Issue here: #704