Inject the shared ViewPanel stylesheet into document.head if
it isn't already there. Idempotent across calls and across
separate ViewPanel modules in different bundles (guarded by a
<style id> lookup as well as a module-local flag).
Exported so callers that render xkt-vp-panel chrome without
constructing a ViewPanel instance (e.g. the pinned-cell
wrappers in ViewManager) can still pull the stylesheet in —
otherwise their buttons render unstyled because the constructor's
own injectStylesOnce() call never fires.
Inject the shared ViewPanel stylesheet into
document.headif it isn't already there. Idempotent across calls and across separate ViewPanel modules in different bundles (guarded by a<style id>lookup as well as a module-local flag).Exported so callers that render
xkt-vp-panelchrome without constructing aViewPanelinstance (e.g. the pinned-cell wrappers inViewManager) can still pull the stylesheet in — otherwise their buttons render unstyled because the constructor's owninjectStylesOnce()call never fires.