diff --git a/src/lib/actions/tooltip.ts b/src/lib/actions/tooltip.ts
index 717a342..d1a7b16 100644
--- a/src/lib/actions/tooltip.ts
+++ b/src/lib/actions/tooltip.ts
@@ -12,6 +12,26 @@ function getTooltipEl(): HTMLDivElement {
   return tooltipEl;
 }
 
+function clamp(val: number, min: number, max: number) {
+  return Math.min(Math.max(val, min), max);
+}
+
+function positionTooltip(el: HTMLDivElement, cx: number, cy: number) {
+  const margin = 6;
+  // Start with ideal position: just right of and below cursor
+  let x = cx + 12;
+  let y = cy + 16;
+  // Measure after content is set (element is in DOM, opacity-hidden is fine)
+  const w = el.offsetWidth;
+  const h = el.offsetHeight;
+  // Shift left/right only as much as needed
+  x = clamp(x, margin, window.innerWidth - w - margin);
+  // Shift up/down only as much as needed
+  y = clamp(y, margin, window.innerHeight - h - margin);
+  el.style.left = `${x}px`;
+  el.style.top = `${y}px`;
+}
+
 export function tooltip(node: HTMLElement, text: string) {
   let timer: ReturnType<typeof setTimeout> | null = null;
   let currentText = text;
@@ -22,8 +42,8 @@ export function tooltip(node: HTMLElement, text: string) {
     if (!currentText) return;
     const el = getTooltipEl();
     el.textContent = currentText;
-    el.style.left = `${mouseX + 12}px`;
-    el.style.top = `${mouseY + 16}px`;
+    // Position before making visible — offsetWidth/Height valid since el is in DOM
+    positionTooltip(el, mouseX, mouseY);
     el.classList.add(TOOLTIP_VISIBLE_CLASS);
   }
 
@@ -38,8 +58,7 @@ export function tooltip(node: HTMLElement, text: string) {
     mouseX = e.clientX;
     mouseY = e.clientY;
     if (!tooltipEl?.classList.contains(TOOLTIP_VISIBLE_CLASS)) return;
-    tooltipEl.style.left = `${mouseX + 12}px`;
-    tooltipEl.style.top = `${mouseY + 16}px`;
+    positionTooltip(tooltipEl, mouseX, mouseY);
   }
 
   function leave() {
@@ -56,6 +75,7 @@ export function tooltip(node: HTMLElement, text: string) {
       currentText = newText;
       if (tooltipEl?.classList.contains(TOOLTIP_VISIBLE_CLASS)) {
         tooltipEl.textContent = newText;
+        positionTooltip(tooltipEl, mouseX, mouseY);
       }
     },
     destroy() {