feat: Use custom character to draw editor rulers

[codingjerk]

This PR allows selecting custom character to draw editor rulers and it contains fixes and improvements on PR #9256

Differences from previous PR

1. No unnecessary formatting changes
2. No unnecessary string allocation for every character drawn
3. Separate theme key (ui.virtual.ruler.char) if editor.ruler-char is set to allow using different styles for "background ruler" and "character ruler"
4. Ruler character is drawn under the text

Screenshot

Default theme with config

[editor]
  rulers = [ 40, 60 ]
  ruler-char = "¦"

[codingjerk]

I believe I also need to add theme key ui.virtual.ruler.char to every theme in helix.

I believe reasonable value for it is the same as ui.virtual.indent-guide. Using bg from ui.virtual.ruler as fg doesn't work well — in some themes it's pretty hard to see "character rulers" since they're thinner.

I'll wait for reviewers to approve this approach, before adding new key to every theme.

[baldwindavid]

@codingjerk Thanks for this; a real big win here is fixing the issue of the disappearing cursor over a ruler.

I read about some of the constraints and complications in #9256 and see that it's not completely straightforward how to approach.

Not a reviewer at all, but I do wonder if an option might be to introduce a couple settings:

Settings

| `ruler-style` | `bg` displays the ruler as a background color, while `char` displays the configured `ruler-char` | `bg` |
| `ruler-char`  | Specifies the character used to display the rulers when `ruler-style` is `char` | `|` |

The default style would be background so as not to break current settings. The default ruler character is set to |, but will only display if the ruler-style is set to char.

Theme Changes

Perhaps an additional theme key is then not necessary. Instead, the bg dictates the bg ruler-style just as it does now and fg dictates the char ruler-style.

"ui.virtual.ruler" = { bg = "cursorline", fg = "cursorline" }

[codingjerk]

Hi @baldwindavid, I’m glad you found it useful! Regarding your thoughts:

"ui.virtual.ruler" = { bg = "cursorline", fg = "cursorline" }

I like this idea and originally considered it, but it would break the current behavior where both background and foreground colors can be set for the ruler. For example, you can highlight characters under the ruler in red:

It would also slightly complicate the code, as this PR just selects the corresponding theme key, whereas your solution requires extracting the style from within the style value. While it’s not a huge issue, it’s still something to note.

That said, I think it could be worth it, as it simplifies configuration and theming a bit. Thanks for bringing it up.

I hope the maintainers can comment which solution is better, and I’d be happy to either keep the current approach or implement yours.

ruler-style

This is an interesting idea — it would make switching the ruler style easier since the user wouldn't need to select a character manually if they’re fine with the default. It’s also more explicit, and I like it. This solution also doesn't have any significant tradeoffs, I believe. Introducing two more options is okay, since it will simplify ruler-char making it char instead of Option<char>.

Both ideas are independent, so I will update my PR to include the second idea (the ruler-style option) while waiting for the maintainers' feedback on the first (using single theme key).

[codingjerk]

I added commit 4acdbe3 with separate ruler-style option. LGTM now.

Waiting for maintainers to comment on themes