Highlight current narrow (rebased/simplified/extended) #1244

[neiljp]

What does this PR do?

This is a rebase and minor adjustment to get #1244 working against current main.

This seems to work well, highlighting the stream in the left panel when in the associated narrow, and with some simple extensions also shows the top-left buttons highlighted when changing to the appropriate narrow.

This still has no tests and may benefit from refactoring - digging into the view to toggle button highlighting is not great.

A final version of something like this would fix #516, like the original.

Tested?

• [x] Manually
• [ ] Existing tests (adapted, if necessary)
• [ ] New tests added (for any new behavior)
• [ ] Passed linting & tests (each commit)

Notes & Questions

I'm particularly interested in the style per theme - this can be changed later, but we can get some reasonable values to begin with.

Visual changes

[zulipbot]

Hello @neiljp, it seems like you have referenced #516 in your pull request description, but you have not referenced them in your commit message description(s). Referencing an issue in a commit message automatically closes the corresponding issue when the commit is merged, which makes the issue tracker easier to manage.

Please run git commit --amend in your command line client to amend your commit message description with Fixes #516..

An example of a correctly-formatted commit:

commit fabd5e450374c8dde65ec35f02140383940fe146
Author: zulipbot
Date:   Sat Mar 18 13:42:40 2017 -0700

    pull requests: Check PR commits reference when issue is referenced.

    Fixes #51.

To learn how to write a great commit message, please refer to our guide.

[neiljp]

I've closed the PRs leading up to this work, but note there may be useful comments in those PRs for anyone taking this forward.