Sticky headers for BCD table, on desktop

On desktop with sufficient browser height (determined by a media
query on `(min-height: $screen-height-place-limit)`), make the
headers of the BCD table sticky against the bottom of the sticky
page header.

Signed-off-by: FeRD (Frank Dana) <ferdnyc@gmail.com>

client/src/document/ingredients/browser-compatibility-table/index-desktop-sticky.scss

@@ -0,0 +1,23 @@
+// Sticky BCD table headers, on sufficiently tall screens
+@use "../../../ui/vars" as *;
+
+@media (min-height: $screen-height-place-limit) {
+  .bc-table {
+    position: relative;
+
+    thead {
+      --offset: var(--sticky-header-with-actions-height);
+      --thead-z-index: calc(var(--z-index-main-header) - 100);
+      background-color: var(--background-primary);
+
+      position: sticky;
+      top: var(--offset);
+      z-index: var(--thead-z-index);
+    }
+  }
+
+  .table-container {
+    overflow-x: auto;
+    overflow-y: visible;
+  }
+}

client/src/document/ingredients/browser-compatibility-table/index.scss

@@ -387,6 +387,7 @@ dl.bc-notes-list {
 @import "./index-desktop";
 @import "./index-desktop-md";
 @import "./index-desktop-xl";
+@import "./index-desktop-sticky";
 
 .bcd-cell-text-wrapper {
   display: flex;