fix: tooltip positioning without rAF — measure synchronously via ready class

data/units.json

@@ -272,7 +272,7 @@
       "id": "lengths-and-distance",
       "label": "Lengths and Distance",
       "baseUnitId": "mm",
-      "toUniversal": 14.2332177486,
+      "toUniversal": 1,
       "alwaysShowLabel": false,
       "hidden": false
     }

src/app.css

@@ -469,6 +469,10 @@
 .hu-tooltip--visible {
   opacity: 1;
 }
+.hu-tooltip--ready {
+  opacity: 1;
+  transition: none;
+}
 [data-theme=dan] .hu-tooltip {
   background: #000;
   color: #ffff00;

src/lib/actions/tooltip.ts

@@ -8,6 +8,8 @@
  */
 
 const TOOLTIP_CLASS = 'hu-tooltip';
+const TOOLTIP_VISIBLE_CLASS = 'hu-tooltip--visible';
+const TOOLTIP_READY_CLASS = 'hu-tooltip--ready';
 
 // Shared tooltip element — one per page, repositioned as needed.
 let tooltipEl: HTMLDivElement | null = null;
@@ -21,32 +23,16 @@ function getTooltipEl(): HTMLDivElement {
   return tooltipEl;
 }
 
-export function tooltip(node: HTMLElement, text: string) {
+/**
-  let timer: ReturnType<typeof setTimeout> | null = null;
+ * Position the tooltip near (cx, cy), shifting up only as much as needed
-  let currentText = text;
+ * to keep it within the viewport. Requires the element to already be in
-
+ * the DOM with its content set so offsetWidth/offsetHeight are accurate.
-  function show(e: MouseEvent) {
+ */
-    if (!currentText) return;
-    const el = getTooltipEl();
-    el.textContent = currentText;
-
-    // Position off-screen first so we can measure dimensions after render
-    el.style.left = '-9999px';
-    el.style.top = '-9999px';
-    el.classList.add('hu-tooltip--visible');
-
-    // Use rAF to let the browser lay out the element before measuring
-    requestAnimationFrame(() => {
-      position(el, e.clientX, e.clientY);
-    });
-  }
-
 function position(el: HTMLDivElement, cx: number, cy: number) {
   const margin = 8;
   const w = el.offsetWidth;
   const h = el.offsetHeight;
   const x = Math.min(cx + 12, window.innerWidth - w - margin);
-    // Place below cursor; shift up only as much as needed to fit
   let y = cy + 20;
   const overflow = y + h + margin - window.innerHeight;
   if (overflow > 0) y -= overflow;
@@ -55,14 +41,37 @@ export function tooltip(node: HTMLElement, text: string) {
   el.style.top = `${y}px`;
 }
 
+export function tooltip(node: HTMLElement, text: string) {
+  let timer: ReturnType<typeof setTimeout> | null = null;
+  let currentText = text;
+
+  function show(cx: number, cy: number) {
+    if (!currentText) return;
+    const el = getTooltipEl();
+    el.textContent = currentText;
+
+    // Make visible but suppress the opacity transition while we measure+position.
+    // --ready class sets opacity:1 with no transition; --visible adds the fade-in.
+    el.classList.remove(TOOLTIP_VISIBLE_CLASS);
+    el.classList.add(TOOLTIP_READY_CLASS);
+
+    // offsetWidth/offsetHeight are now accurate (element is in DOM, display not none)
+    position(el, cx, cy);
+
+    // Swap to the visible class to re-enable the fade transition
+    el.classList.remove(TOOLTIP_READY_CLASS);
+    el.classList.add(TOOLTIP_VISIBLE_CLASS);
+  }
+
   function enter(e: MouseEvent) {
     if (!currentText) return;
-    timer = setTimeout(() => show(e), 500);
+    const cx = e.clientX;
+    const cy = e.clientY;
+    timer = setTimeout(() => show(cx, cy), 500);
   }
 
   function move(e: MouseEvent) {
-    // Keep tooltip near cursor while hovering
+    if (!tooltipEl?.classList.contains(TOOLTIP_VISIBLE_CLASS)) return;
-    if (!tooltipEl?.classList.contains('hu-tooltip--visible')) return;
     position(getTooltipEl(), e.clientX, e.clientY);
   }
 
@@ -71,7 +80,11 @@ export function tooltip(node: HTMLElement, text: string) {
       clearTimeout(timer);
       timer = null;
     }
-    tooltipEl?.classList.remove('hu-tooltip--visible');
+    const el = tooltipEl;
+    if (el) {
+      el.classList.remove(TOOLTIP_VISIBLE_CLASS);
+      el.classList.remove(TOOLTIP_READY_CLASS);
+    }
   }
 
   node.addEventListener('mouseenter', enter);
@@ -81,7 +94,7 @@ export function tooltip(node: HTMLElement, text: string) {
   return {
     update(newText: string) {
       currentText = newText;
-      if (tooltipEl?.classList.contains('hu-tooltip--visible')) {
+      if (tooltipEl?.classList.contains(TOOLTIP_VISIBLE_CLASS)) {
         tooltipEl.textContent = newText;
       }
     },
@@ -90,7 +103,11 @@ export function tooltip(node: HTMLElement, text: string) {
       node.removeEventListener('mouseenter', enter);
       node.removeEventListener('mousemove', move);
       node.removeEventListener('mouseleave', leave);
-      tooltipEl?.classList.remove('hu-tooltip--visible');
+      const el = tooltipEl;
+      if (el) {
+        el.classList.remove(TOOLTIP_VISIBLE_CLASS);
+        el.classList.remove(TOOLTIP_READY_CLASS);
+      }
     }
   };
 }