Frontend & browser·Performance and Core Web Vitals
the profiler flags a purple block with a warning triangle where you asked for an element's width.
Forced synchronous layout
Also calledforced reflow, layout trashing
Reading a geometric property while style or layout changes are still pending, which forces the browser to run layout immediately rather than at its scheduled point. A single one is cheap; it is only expensive when it happens repeatedly. The list of triggering properties is long and unintuitive — `offsetTop`, `getBoundingClientRect`, `scrollTop`, `getComputedStyle` — which is why the profiler's warning is more reliable than memory.