feat: add offcanvas() panels

[gadenbuie]

Closes #1194

Summary

Adds offcanvas() plus server verbs show_offcanvas()/hide_offcanvas()/toggle_offcanvas() for Bootstrap offcanvas panels (sliding side/top/bottom drawers). offcanvas() returns a deferred bslib_offcanvas object (same pattern as toast()/sidebar()) supporting three usage modes, selected by whether trigger/id are supplied. State and lifecycle are governed by id: an explicit id gets a live input$<id> logical binding and persists in the DOM across hide/show; an anonymous panel self-destructs (unbinds + removes) on hide. Client-side binding is a <bslib-offcanvas> web component wrapping bootstrap.Offcanvas, registered alongside the existing popover/switch web components; show_offcanvas() rides a custom message (insert-if-absent + show, required since no client binding exists yet for server-only panels) while hide/toggle ride sendInputMessage. Styling lives in inst/components/scss/offcanvas.scss, auto-picked-up by the existing SCSS glob (no new dependency registration needed).

Verification

(a) Trigger + panel in the UI -- clicking the trigger reveals the panel client-side, no server round-trip:

offcanvas(
  "Panel content here.",
  title = "Settings",
  trigger = actionButton("open", "Open panel")
)

(b) Id'd panel in the UI, controlled from the server:

ui <- page_fillable(
  offcanvas("Panel content here.", title = "Settings", id = "oc"),
  actionButton("open", "Open panel"),
  verbatimTextOutput("state")
)
server <- function(input, output, session) {
  observeEvent(input$open, toggle_offcanvas("oc"))
  output$state <- renderText(paste("input$oc =", input$oc))
}

(c) Server-defined panel, revealed on demand (no UI declaration needed):

server <- function(input, output, session) {
  observeEvent(input$open, {
    show_offcanvas(offcanvas("Panel content here.", title = "Settings"))
  })
}

In each case the panel slides in from the right edge of the viewport (default placement); input$oc (mode b) flips to TRUE/FALSE as it's shown/hidden.

[Copilot]

Pull request overview

Adds a new Bootstrap 5 offcanvas component to bslib, including an R API (offcanvas()) and Shiny server controls (show_offcanvas(), hide_offcanvas(), toggle_offcanvas()), backed by new client-side web components and message handlers plus styling and documentation updates.

Changes:

• Introduces offcanvas() + server verbs with lifecycle semantics (persistent vs temporary panels).
• Adds a <bslib-offcanvas> web component (Shiny input binding) and a bslib.show-offcanvas custom message handler for server-defined insertion.
• Adds SCSS styling, docs/reference entries, NEWS entry, and comprehensive test coverage.

Reviewed changes

Copilot reviewed 12 out of 25 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
R/offcanvas.R	New offcanvas API, tag rendering, trigger wiring, preview, and server verbs.
tests/testthat/test-offcanvas.R	Unit tests for constructor defaults, tag output, a11y wiring, sizing, and server verb messaging.
srcts/src/components/webcomponents/offcanvas.ts	New <bslib-offcanvas> Shiny input web component wrapping Bootstrap Offcanvas.
srcts/src/components/webcomponents/index.ts	Registers the new web component.
srcts/src/components/offcanvas.ts	Adds bslib.show-offcanvas custom message handler to insert/show server-defined panels.
srcts/src/components/index.ts	Ensures the new offcanvas message handler module is loaded.
inst/components/scss/offcanvas.scss	Styling for offcanvas title and footer.
inst/components/dist/web-components.js	Built web-components bundle update to include <bslib-offcanvas>.
inst/components/dist/web-components.min.js	Minified web-components bundle update.
inst/components/dist/components.js	Built components bundle update to include bslib.show-offcanvas handler.
inst/components/dist/components.min.js	Minified components bundle update.
inst/components/dist/components.css	Built CSS update to include offcanvas styles.
pkgdown/_pkgdown.yml	Adds Offcanvas reference section to pkgdown site.
man/offcanvas.Rd	Generated documentation for offcanvas().
man/show_offcanvas.Rd	Generated documentation for server verbs.
NEWS.md	Adds a NEWS entry for the new feature.
NAMESPACE	Exports new functions and S3 methods.
DESCRIPTION	Adds offcanvas.R to collate list.
man/navbar_options.Rd	Minor formatting tweak in docs.
inst/examples-shiny/code-editor/examples/r.r	Fixes assignment operator in an example snippet.

Files not reviewed (2)

• man/offcanvas.Rd: Generated file
• man/show_offcanvas.Rd: Generated file

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[cpsievert]

Nice, thanks! Copilot left some nitpicky feedback, but the overall API here looks great.