fix: tup-488 scrollable table border (#265)

- **fixed** outdated comment
- **added** top-of-rows border via thead
- **changed** top-of-rows border via tbody to only be for headless table
- **added** comments to explain unique cell borders
- **changed** comment to be consistent

src/lib/_imports/elements/table--basic.css

@@ -8,7 +8,7 @@ table,
   --cell-line-height: 1.1;
 
   /* To prevent borders from being hidden */
-  /* NOTE: Specifically, the right border of pinned columns (narrow windows) */
+  /* SEE: TABLE WITH A <thead> */
   border-collapse: separate; /* overwrite Bootstrap */
   border-spacing: 0;
 
@@ -19,9 +19,19 @@ table,
   display: table; /* to mimic shrink-wrap width of real table */
 }
 
-/* To add top border to tbody (unless nested) */
-/* FAQ: Added to cell, not row, because `border-collapse: separate` */
-tbody > tr:first-child > :is(td, th),
+/* TABLE WITH A <thead> */
+/* FAQ: To __avoid__ <thead> border scrolling away on an `o-fixed-header-table`:
+        - border must be on the cells
+        - table must have `border-collapse: separate` */
+thead tr:last-child :is(td, th) {
+  border-bottom: var(--global-border-width--normal) solid var(--global-color-primary--dark);
+}
+/* TABLE WITH NO <thead> */
+/* FAQ: To __avoid__ styling a __nested__ table, must use direct selector `>`  */
+/* FAQ: To __ensure__ border is __visible__:
+        - border must be on cell
+          (because table has `border-collapse: separate`) */
+tbody:not(thead + tbody) > tr:first-child > :is(td, th),
 :--s-paragraph-table {
   border-top: var(--global-border-width--normal) solid var(--global-color-primary--dark);
 }

src/lib/_imports/elements/table.cms.css

@@ -9,7 +9,8 @@ table,
   border-bottom: var(--global-border-width--thick) solid var(--global-color-primary--dark);
 }
 
-/* To thicken top border of headless table */
+/* TABLE WITH NO <thead> */
+/* To thicken top border */
 tbody:not(thead + tbody) > tr:first-child > :is(td,th),
 :--s-paragraph-table {
   border-top-width: var(--global-border-width--thick);