Reserve Stable Space for Ads and Third-Party Embeds

.embed-slot { min-block-size: 0; }

.embed-slot { min-block-size: 160px; }

Root cause

Late content in normal flow needs space. If the initial slot is empty and has zero height, inserting a panel increases the slot’s block size and moves every following element. The movement comes from the page’s own geometry, not from whether the inserted content is an ad, video, map, or project widget.

The fixed fixture gives the slot the exact height of this controlled panel. It does not imply that every production ad should use 160 pixels. A real integration may need format-specific sizes, responsive aspect ratios, breakpoint rules, or a conservative minimum based on measured delivery data.

Empty and unfilled slots

Space reservation creates a tradeoff when no content arrives. A publisher can retain the region, collapse it before it enters the viewport, or choose a smaller expected size. Each choice should be tested with actual fill behavior. Hiding an oversized blank area by placing content over it is not a layout-stability fix.

Measurement support

Every browser records slot height before and after insertion plus the downstream article’s movement. Chromium additionally records Layout Instability API entries that do not follow recent input. The fixture never requests third-party content.

When this fix is not enough

Content that becomes taller than its reservation can still shift the page. Content rendered inside an iframe can also shift internally without moving the outer document. Test both the outer slot and the embedded experience, and do not infer production revenue or viewability from this geometry fixture.

Regression condition

At 320 and 375 CSS pixels, inserting the project panel into the failing slot must move the following article by 160 pixels. The fixed slot must begin and end at 160 pixels and produce zero target movement. Both states must have zero horizontal page overflow.