content/static/css: make .Dialog's position fixed

Add `position:fixed` in `.Dialog` class to make appearance of both
`.JumpDialog` and `ShortcutsDialog` consistent

Fixes golang/go#43877

Change-Id: I9a1940455251ae057f53e1dbb210854e4cd203a1
GitHub-Last-Rev: 30611f1b2111d5f8d1ecd748e659688186b20bd0
GitHub-Pull-Request: golang/pkgsite#22
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/286092
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/content/static/css/stylesheet.css b/content/static/css/stylesheet.css
index 07192c3..9a8824f 100644
--- a/content/static/css/stylesheet.css
+++ b/content/static/css/stylesheet.css
@@ -1563,6 +1563,7 @@
 
 .Dialog {
   padding: 0;
+  position: fixed;
   border: 0.0625rem solid rgba(0, 0, 0, 0.2);
   border-radius: 6px;
   box-shadow: 0 0.3125rem 0.9375rem rgba(0, 0, 0, 0.5);
@@ -1587,7 +1588,6 @@
 }
 
 .JumpDialog {
-  position: fixed;
   width: 25rem;
 }
 .JumpDialog-body {