The Frame Fight: Why Your Nav Bar Argues With Everything You Move on a Page — and What That Means for Your Next A/B Test

A plain-language paper for working designers reporting one measured physics result from a 17-experiment certification arc: design changes interact mostly through frame elements — the nav bar and page layout. When two design elements change at once, the combined effect is usually slightly less than the sum of the parts, but when a frame element is involved, it can be greater. We ranked all change-pairs by interaction strength on controlled test pages and on re-creations of five real pairs from published case studies; the strongest interactions involved frame elements in every case, replicated under different render conditions. Practical implications: one change per test is physics, not dogma; frame changes deserve an interaction warning label; per-element credit should be split with interaction terms (Shapley values), not by simple attribution. The paper makes no winner-prediction claim and states its limits in a dedicated box.