One of the most difficult programming challenges I have encountered is calculating and displaying automatic drag and drop alignment guidelines/rulers (like those in Figma and PowerPoint that show up when you drag two element close to each other or near certain ratios) efficiently. They are not documented anywhere despite being a very common pattern. Most drag and drop libraries and framework don't have out of the box support for it as it requires deep low level integration.
On a related note, the tab stop/indent marker/ruler in MS Word (and the related ruler/guidelines in Photoshop and others) are sorely missing in the generally available UI elements. For text on the web I get it, since it's all markdown and/or html, not exactly WYSIWYG-emulating-paper. Guidelines for the purposes of snap alignment are relatively easy to implement and have tons of use, but I have not seen any good ones in the wild yet.
