Fixes #1377.

Problem

On the landing page, the horizontal publication card's .pub-download-links row (PDF/Cite/project chips) is hidden with opacity:0, which still occupies layout space. That row can extend past the card's 129px box, and the landing grid is a flex-wrap row, so a card's revealed-on-hover chips can sit over the title of the card in the row below (or, in the last row, over "SEE ALL PAPERS"). At the default stacking order the chips painted behind that content and looked tangled — the reported "sometimes the chips overlap" (the "sometimes" being the variable stack height: author count, chip count, title length).

Fix

On :hover/:focus-within, give the revealed chips position: relative and z-index: 30 so they paint cleanly on top of whatever is below them instead of tangling behind it.

.pub-column-horiz-layout:hover .pub-download-links,
.pub-column-horiz-layout:focus-within .pub-download-links {
  opacity: 1;
  position: relative;
  z-index: 30; /* clears thumbnail (10) & award banner (20) */
}

• z-index is applied only on hover/focus, so the invisible (opacity:0) layer never sits over — and swallows clicks meant for — the card below. At-rest behavior is unchanged.
• 30 clears the existing thumbnail (z-index:10) and award banner (z-index:20) stacking.

This fixes the layering (chips legible, on top); it intentionally doesn't reserve extra space, so the transient on-hover overlap itself remains by design.

Verification

• Real :hover on the chip-heavy "At the Intersection…" card: revealed chips now sit cleanly on top of the content below (before/after crop to be attached).
• a11y: no change to colors/contrast, DOM order, or keyboard reveal (:focus-within preserved). Pa11y landing-page findings are the pre-existing media-less color-contrast noise; none reference the touched selectors.
• No Django test applies (pure CSS render bug, not reachable via the Python suite).

Screenshot

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