fix: hide conditionalPanel() on initial render to prevent flash (#4377)

* fix: hide conditionalPanel on initial render to prevent flash (#3505)

conditionalPanel renders visible HTML, but the JS that evaluates the
condition doesn't run until the WebSocket connects. This causes a brief
flash of content that should be hidden.

Fix by using CSS to hide `.shiny-panel-conditional` by default, and
toggling a `--active` modifier class in JS when the condition is true.
This also avoids jQuery's `.show()` setting `display: block`, which
would conflict with BS5's `display: contents` pass-through rule.

* `npm run build` (GitHub Actions)

* Add NEWS entry for conditionalPanel flash fix (#3505)

* Target [data-display-if] in CSS instead of .shiny-panel-conditional

The JS targets all [data-display-if] elements, so the CSS should match.
This ensures hand-crafted HTML with data-display-if (without the
.shiny-panel-conditional class) also gets the hidden-by-default behavior.

* Extract CSS class name into a single const

* Rename CSS class to shiny-conditional--shown

Drops "panel" since the class applies to any [data-display-if] element,
not just conditionalPanel() output. Uses "shown" to mirror the existing
show/shown/hide/hidden events and avoid confusion with Bootstrap's
"active" class.

* Use native classList API instead of jQuery for class toggling

* Revert "Use native classList API instead of jQuery for class toggling"

This reverts commit 2c077b9.

* Move conditionalPanel news item into Improvements section

---------

Co-authored-by: cpsievert <cpsievert@users.noreply.github.com>

Author: cpsievert

NEWS.md

@@ -12,6 +12,9 @@
   for code that needs to trigger theme clientdata refreshes after changing
   surrounding visual theme context. (#3682)
 
+* `conditionalPanel()` no longer briefly flashes its contents on app start
+  when the condition is initially `FALSE`. (#3505)
+
 # shiny 1.13.0
 
 ## New features