Add magnification when editing pet appearance

I couldn't find a library for this functionality that didn't require
jQuery, and I don't want to be adding *more* jQuery requirements. So, I
decided to throw together my own!

The `<magic-magnifier>` component copies its contents into a "lens"
element, then uses basic JS to track mouse position, then uses CSS to
move the lens and its contents into a helpful position.

One thing I noticed here is that the zoom is a bit crunchy because
we're using PNG images, and it's hard to zoom in even further than we
already are. I might try switching this UI to use the SVG images by
default instead?

app/assets/javascripts/magic-magnifier.js

@@ -0,0 +1,33 @@
+class MagicMagnifier extends HTMLElement {
+	connectedCallback() {
+		setTimeout(() => this.#attachLens(), 0);
+		this.addEventListener("mousemove", this.#onMouseMove);
+	}
+
+	#attachLens() {
+		const lens = document.createElement("magic-magnifier-lens");
+		lens.inert = true;
+		lens.useContent(this.children);
+		this.appendChild(lens);
+	}
+
+	#onMouseMove(e) {
+		const lens = this.querySelector("magic-magnifier-lens");
+		const rect = this.getBoundingClientRect();
+		const x = e.pageX - rect.left;
+		const y = e.pageY - rect.top;
+		lens.style.setProperty("--magic-magnifier-x", x + "px");
+		lens.style.setProperty("--magic-magnifier-y", y + "px");
+	}
+}
+
+class MagicMagnifierLens extends HTMLElement {
+	useContent(contentNodes) {
+		for (const contentNode of contentNodes) {
+			this.appendChild(contentNode.cloneNode(true));
+		}
+	}
+}
+
+customElements.define("magic-magnifier", MagicMagnifier);
+customElements.define("magic-magnifier-lens", MagicMagnifierLens);

app/assets/javascripts/pet_states/support-outfit-viewer.js

@@ -13,22 +13,26 @@ class SupportOutfitViewer extends HTMLElement {
 		if (!e.target.matches("tr")) return;
 
 		const id = e.target.querySelector("[data-field=id]").innerText;
-		const layer = this.querySelector(
+		const layers = this.querySelectorAll(
 			`outfit-viewer [data-asset-id="${CSS.escape(id)}"]`,
 		);
+		for (const layer of layers) {
 			layer.setAttribute("highlighted", "");
 		}
+	}
 
 	// When a row is unhovered, unhighlight its corresponding outfit viewer layer.
 	#onMouseLeave(e) {
 		if (!e.target.matches("tr")) return;
 
 		const id = e.target.querySelector("[data-field=id]").innerText;
-		const layer = this.querySelector(
+		const layers = this.querySelectorAll(
 			`outfit-viewer [data-asset-id="${CSS.escape(id)}"]`,
 		);
+		for (const layer of layers) {
 			layer.removeAttribute("highlighted");
 		}
+	}
 
 	// When clicking a row, redirect the click to the first link.
 	#onClick(e) {

app/assets/stylesheets/application/magic-magnifier.sass

@@ -0,0 +1,40 @@
+magic-magnifier
+	position: relative
+
+	&:not(:hover)
+		magic-magnifier-lens
+			display: none
+
+magic-magnifier-lens
+	width: var(--magic-magnifier-lens-width, 100px)
+	height: var(--magic-magnifier-lens-height, 100px)
+	overflow: hidden
+	border-radius: 100%
+
+	background: white
+	border: 2px solid black
+	box-shadow: 3px 3px 3px rgba(0, 0, 0, .5)
+
+	position: absolute
+	left: var(--magic-magnifier-x, 0px)
+	top: var(--magic-magnifier-y, 0px)
+
+	> *
+		// Translations are applied in the opposite of the order they're specified.
+		// So, here's what we're doing:
+		//
+		// 1. Translate the content left by --magic-magnifier-x and up by
+		//    --magic-magnifier-y, to align the target location with the lens's
+		//    top-right corner.
+		// 2. Zoom in by --magic-magnifier-scale.
+		// 3. Translate the content right by half of --magic-magnifier-lens-width,
+		//    and down by half of --magic-magnifier-lens-height, to align the
+		//    target location with the lens's center.
+		//
+		// Note that it *is* possible to specify transforms relative to the center,
+		// rather than the top-left corner—this is in fact the default!—but that
+		// gets confusing fast with scale in play. I think this is easier to reason
+		// about with the top-left corner in terms of math, and center it after the
+		// fact.
+		transform: translateX(calc(var(--magic-magnifier-lens-width, 100px) / 2)) translateY(calc(var(--magic-magnifier-lens-height, 100px) / 2)) scale(var(--magic-magnifier-scale, 2)) translateX(calc(-1 * var(--magic-magnifier-x, 0px))) translateY(calc(-1 * var(--magic-magnifier-y, 0px)))
+		transform-origin: left top

app/assets/stylesheets/pet_states/support-outfit-viewer.sass

@@ -37,3 +37,8 @@ support-outfit-viewer
 				cursor: zoom-in
 				&:hover
 					background: $module-bg-color
+
+	magic-magnifier
+		--magic-magnifier-lens-width: 100px
+		--magic-magnifier-lens-height: 100px
+		--magic-magnifier-scale: 2.5

app/views/pet_states/_support_outfit_viewer.html.haml

@@ -1,5 +1,7 @@
 = content_tag "support-outfit-viewer", **html_options do
+	%magic-magnifier
 		= outfit_viewer outfit
+
 	%table
 		%thead
 			%tr

app/views/pet_states/edit.html.haml

@@ -45,11 +45,13 @@
 
 - content_for :stylesheets do
 	= stylesheet_link_tag "application/breadcrumbs"
+	= stylesheet_link_tag "application/magic-magnifier"
 	= stylesheet_link_tag "application/outfit-viewer"
 	= stylesheet_link_tag "application/support-form"
 	= stylesheet_link_tag "pet_states/support-outfit-viewer"
 	= page_stylesheet_link_tag "pet_states/edit"
 
 - content_for :javascripts do
+	= javascript_include_tag "magic-magnifier"
 	= javascript_include_tag "outfit-viewer"
 	= javascript_include_tag "pet_states/support-outfit-viewer"