static: display output container for runnable examples

Examples on pkg.go.dev that are runnable but lack pre-defined output
previously did not display the output container when they were executed.
This was because the template only rendered the output <pre> block
conditionally if .Output was present.

Modified static/doc/example.tmpl to always render the output container
for runnable examples, hiding it by default if no initial output exists.
Updated the PlaygroundExampleController in static/shared/playground/playground.ts
to manage the visibility of this container when execution results are
received.

Added regression tests:
- A client-side unit test in static/shared/playground/playground.test.ts
- A server-side template test in internal/godoc/dochtml/dochtml_test.go

Fixes golang/go#75062

Change-Id: I37b3922c99d956ff1b139834e4625eb8d61b84cd
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/752184
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ethan Lee <ethanalee@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/static/doc/example.tmpl b/static/doc/example.tmpl
index ab54b246..ece580cf 100644
--- a/static/doc/example.tmpl
+++ b/static/doc/example.tmpl
@@ -15,7 +15,10 @@
       <p><a class="Documentation-examplesPlay" href="{{.}}">Open in Go playground »</a></p>{{"\n" -}}
       {{- end -}}
       {{render_code .Example}}{{"\n" -}}
-      {{- if .Output -}}<pre><span class="Documentation-exampleOutputLabel">Output:</span>{{"\n\n"}}<span class="Documentation-exampleOutput">{{- .Output -}}</span></pre>{{"\n" -}}{{- end -}}
+      {{- if or .Output .Play -}}
+        <pre class="Documentation-exampleOutputContainer js-exampleOutputContainer"
+          {{- if not .Output}} hidden{{end}}><span class="Documentation-exampleOutputLabel">Output:</span>{{"\n"}}<span class="Documentation-exampleOutput">{{- .Output -}}</span></pre>{{"\n" -}}
+      {{- end -}}
     </div>{{"\n" -}}
     {{- if .Play -}}
       <div class="Documentation-exampleButtonsContainer">