All of the below properties or methods, when requested/called in JavaScript, will force the browser to synchronously calculate the style and layout. This is also called reflow or layout thrashing, and is common performance bottleneck.

• elem.offsetLeft;, elem.offsetTop;, elem.offsetWidth;, elem.offsetHeight;, elem.offsetParent;
• elem.clientLeft;, elem.clientTop;, elem.clientWidth;, elem.clientHeight;
• elem.getClientRects();, elem.getBoundingClientRect();

• elem.scrollWidth;, elem.scrollHeight;
• elem.scrollLeft;, elem.scrollTop; // also setting them
• elem.scrollIntoView();, elem.scrollIntoViewIfNeeded();
• elem.scrollBy();, elem.scrollTo();

• elem.focus(); // can trigger a *double* forced layout (source)

• elem.computedRole;, elem.computedName;
• elem.innerText; (source)

• range.getClientRects();, range.getBoundingClientRect();

• mouseEvt.layerX;, mouseEvt.layerY;, mouseEvt.offsetX;, mouseEvt.offsetY;

source

window.getComputedStyle(); // will typically force style recalc (source)

window.getComputedStyle(); // will force layout, as well, if one of the following:

1. The element is in a shadow tree
2. There are media queries (viewport-related ones); specifically, one of the following: (source)

• min-width, min-height, max-width, max-height, width, height
• aspect-ratio, min-aspect-ratio, max-aspect-ratio
• device-pixel-ratio, resolution, orientation

1. The property requested is one of the following: (source)

• height, width
• top, right, bottom, left
• margin [-top, -right, -bottom, -left, or shorthand] only if the margin is fixed.
• padding [-top, -right, -bottom, -left, or shorthand] only if the padding is fixed.
• transform, transform-origin, perspective-origin
• translate, rotate, scale
• webkit-filter, backdrop-filter
• motion-path, motion-offset, motion-rotation
• x, y, rx, ry

• doc.scrollingElement; // only forces style

• window.scrollX;, window.scrollY;

• window.innerHeight;, window.innerWidth;

• window.getMatchedCSSRules(); // only forces style

• inputElem.select();, textareaElem.select();

(source)

• Lots & lots of stuff
• …including copying an image to clipboard (source)

• forced layout (and style recalc): updateLayoutIgnorePendingStylesheets - Chromium Code Search
• forced style recalc: updateLayoutTree - Chromium Code Search`