MediaWiki:Gadget-stickyTableHeaders.css

/* keeps table headers stuck to the top of the window,
   useful for long tables where it normally scrolls offscreen
   
   KNOWN PROBLEMS
   - borders disappear due to browser implementation issue (https://bugs.webkit.org/show_bug.cgi?id=128486)
   - double-row headers only work as long as the first <tr> does not stretch its height, since MediaWiki doesn't support <thead>
   - Chrome/Opera/Edge don't support sticky on <thead> anyway (https://caniuse.com/#feat=css-sticky)
   - Safari makes the th elements in <tfoot> scroll down along with the top, if they are made sticky. table > :not(tfoot) fixes this.

   USAGE
   - sticky headers are opt-in using the "sticky-header" class (eg. {| class="wikitable sticky-header")
   - IF using multiple rows of <th>, the first row must not exceed 2.3em in height.
     - If it does, set a custom style="top: 2.4em;" (or something similar) on the second row of <th>
   - To sticky intermediate headers, mark their table-row as |-class="sticky-header" as well.
     - Note: This won't look good when the top header is 2 rows high, but the intermediate rows are 1 row high.
   - don't add sticky headers to tables shorter than roughly 1000px - use your best judgement
*/

table.sticky-header > :not(tfoot) tr:first-child th,
table.sticky-header > :not(tfoot) tr:nth-child(2) th,
tr.sticky-header th {
	position: -webkit-sticky;
	position: sticky;
	height: 2.3em; /* taller contents will override */
	box-sizing: border-box;
	top: -1px; /* chrome has a see-through border? */
	border-bottom: 0;
}

/* Move the second row of sticky headers down exactly the height of a single-line th */
table.sticky-header > :not(tfoot) tr:nth-child(2) th {
	top: calc(2.3em + 1px); /* due to font-size: 0.9em on tables we need +2 px on top of the calc({height}+{top}) */
}

/* fake a bottom border - see known problems */
table.sticky-header > :not(tfoot) tr:first-child th::after,
table.sticky-header > :not(tfoot) tr:nth-child(2) th::after,
tr.sticky-header th::after {
	content: '';
	position: absolute;
	left: 0;
	bottom: -1px; /* merge into existing border */
	width: 100%;
	border-bottom: 1px solid #dcdcdc;
}

/* dark mode border color */
.wgl-darkmode table.sticky-header tr:first-child th::after,
.wgl-darkmode table.sticky-header tr:nth-child(2) th::after,
.wgl-darkmode tr.sticky-header th::after {
	border-bottom-color: #172136;
}


/* move down when sticky page header is enabled */
.wgl-stickyheader.action-view table.sticky-header tr:first-child th,
.wgl-stickyheader.action-view table.sticky-header tr:nth-child(2) th,
.wgl-stickyheader.action-view tr.sticky-header th {
    top: 3em;
}

/* Move the second row of sticky headers down exactly the height of a single-line th */
.wgl-stickyheader.action-view table.sticky-header tr:nth-child(2) th {
	top: calc(3em + 2.3em + 1px); /* due to font-size: 0.9em on tables we need +2 px on top of the calc({height}+{top}) */
}