elm-add-panel

name	elm-add-panel
description	Add a new app-wide "Shared panel" (like Accounts/Starred Posts/Markdown) to the Elm SPA (frontends/elm-spa). Use when asked to add a new global overlay/panel driven from Shared.Model, not page-local state.

Jonline's Elm SPA has a handful of app-wide overlay panels (Accounts Panel, Starred Posts Panel, Admin Panel, Markdown Panel) that live in Shared.Model rather than any one page, so they're available everywhere and survive route changes. There's no central "list of panels" registry — each one is wired by hand into Shared.elm and UI.elm following the same shape. Add a new one the same way.

1. The panel module itself

New file: frontends/elm-spa/src/Shared/<Name>Panel.elm, exposing Model, Msg(..), init, update, view (and whatever helpers callers need).

• update signature depends on whether the panel needs to resolve servers/accounts:
  • Needs AccountsPanel.Model (to look up a Server/Account, make an RPC call, etc.): update : AccountsPanel.Model -> Msg -> Model -> ( Model, Cmd Msg, Maybe AccountsPanel.Msg ). The trailing Maybe AccountsPanel.Msg lets the panel ask Shared.update to forward something to AccountsPanel on its behalf (typically AccountsPanel.AccountRefreshed after a token refresh) — it can't dispatch that itself without importing Shared, which would cycle (Shared imports the panel, not the other way around). See Shared/StarredPostsPanel.elm and Shared/MarkdownPanel.elm.
  • No server/account dependency at all: plain update : Msg -> Model -> Model (no Cmd, no tuple) — see Shared/AdminPanel.elm, the minimal example.
• A panel module can freely import Components.*, Shared.AccountsPanel, Shared.MaybeAccountRequest (none of those import Shared) — but never Shared, UI, or UI.EmittedStylesheet, all of which depend on Shared.Model, which embeds your panel's Model.
• "Always rendered" — the view renders in both open and closed states (never conditionally absent), so opening/closing is a pure CSS opacity/transform transition rather than the element mounting/unmounting. Drive that with UI.Classes.openClosedClass isOpen added as a class alongside "nav-panel" — see any panel's view.

2. Wiring into Shared.elm

Mechanical, copy the shape of the existing StarredPostsPanelMsg/MarkdownPanelMsg branches:

• Add a field to Shared.Model (e.g. yourPanel : YourPanel.Model).
• Add a constructor to Shared.Msg (e.g. YourPanelMsg YourPanel.Msg).
• Initialize it in Shared.init.
• In Shared.update, add a branch that calls the panel's own update, applies any forwarded AccountsPanel.Msg via AccountsPanel.update, and Cmd.batchs both mapped commands together. If the panel has subscriptions, batch those into Shared.subscriptions too (gate on the panel being open if the subscription is expensive/animation-only).

3. Mounting the view in UI.elm

Two patterns depending on how the panel opens:

• Toggled from a nav icon (Starred Posts): add a toggle button + the panel's own render as siblings in headerNav, following starredPostsToggle/starredPostsPanel.
• Opened contextually from a page (Markdown Panel, opened via a page's own Edit/Reply button dispatching Effect.fromShared (Shared.YourPanelMsg (YourPanel.Open ...))): just mount it once, unconditionally, in layout's children list: Html.map toMsg (yourPanel shared) where yourPanel shared = Html.map Shared.YourPanelMsg (YourPanel.view shared.accountsPanel shared.yourPanel).
• Either way, add an entry to sharedBackdrop's panels list so a background tap closes it. That list is ordered nearest-first: a tap only closes the first (topmost, by z-index) open panel in the list, not all of them — put a lower-z-index panel later in the list. Set blurs = True if the panel should block interaction with the rest of the page while open (most editing/login-style panels do); False if it's meant to coexist with using the rest of the app (Starred Posts).

4. CSS

• Reuse the shared .nav-panel base rule (public/style/shared/accounts_panel.css) for position/opacity/pointer-events transition plumbing, then override position/size/z-index in your own public/style/shared/<name>_panel.css. Link the new file in public/index.html — CSS files aren't auto-discovered.
• Existing z-indices: Accounts Panel 20, Starred Posts Panel 25 (deliberately above Accounts). Pick your panel's z-index relative to those based on where it should sit if multiple are open at once.
• To tint an element with a specific server's brand color (e.g. a submit button matching the relevant Post/Group's server) without importing UI/UI.EmittedStylesheet (cycle risk), just add the classes [ frontendHostString, "background-color-primary" ] (or -nav, -primary-5/10/25/50, -primary-background, border-color-primary... — see UI/EmittedStylesheet.elm's doc comment for the full list). UI.EmittedStylesheet.view, mounted once app-wide, already emits a CSS rule per known server keyed by that literal host string as a class — no plumbing needed beyond the class names themselves.