fix: improve sidebar resize handle for fine and coarse pointers

[gadenbuie]

Fixes #1294, both literally and in spirit.

Summary

Improves the sidebar resize handle to work better across pointer types and to avoid conflicts with the sidebar's native scrollbar.

Allow disabling resizable sidebars: Several users have requested the ability to disable sidebar resizing, and this PR makes it possible by adding a resizable argument to sidebar(). The default is TRUE, so app authors who don't want resizable sidebars can now use resizable = FALSE.

Handle width by pointer type:

• Fine pointers (mouse): 8px (down from 12px)
• Coarse pointers (touch): 26px via @media (any-pointer: coarse)

Scrollbar conflict resolution:

The resize handle is positioned just inside the sidebar's outer edge, overlapping the native scrollbar. Previously, clicking the scrollbar would start a resize instead. This is resolved with a midpoint-crossing activation pattern:

• The handle has pointer-events: none by default, so the scrollbar beneath remains fully clickable
• A narrow 5px ::before "trip area" at the sidebar's outer edge detects the mouse crossing the boundary
• When crossed, JS adds a .handle-active class that shows the ew-resize cursor (inherited by ::before) and expands the trip area to 24px (12px per side)
• An engagement-point dismissal mechanism deactivates the handle if the mouse reverses back past where it activated, creating perceived directional bias without asymmetric sizing
• Touch users bypass this entirely — the handle is always interactive with a wide hit area

Screen.Recording.2026-03-16.at.3.05.26.PM.mov

Verification

library(shiny)
library(bslib)

ui <- page_sidebar(
  sidebar = sidebar(
    title = "Sidebar",
    lorem::ipsum(10)
  ),
  "Main content"
)

shinyApp(ui, function(input, output, session) {})

1. Hover over the sidebar's right edge — the resize cursor should NOT appear until the mouse crosses the sidebar boundary
2. Click and drag the sidebar scrollbar — it should scroll, not resize
3. Cross the sidebar edge toward the main area — the resize indicator and cursor should appear
4. Click and drag to resize the sidebar
5. Test on a touch device or with Chrome DevTools device emulation — the handle should be wider and always interactive

[cpsievert]

Should go under a new features section?

[gadenbuie]

It's incremental and minor; I'd call it an "improvement" more than a "new feature"