diff --git a/package.json b/package.json
index 7f16f9d..7238b72 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@tp/helpers",
-  "version": "2.12.1",
+  "version": "2.13.0",
   "description": "",
   "main": "closest.js",
   "scripts": {
diff --git a/position.js b/position.js
index 26d1444..c338073 100644
--- a/position.js
+++ b/position.js
@@ -14,7 +14,13 @@ export const Position = function(superClass) {
       options = Object.assign({
         valign: 'top',
         halign: 'middle',
-        spacing: 0
+        spacing: 0,
+        // When true, clamp against the element's containing-block rect
+        // instead of the viewport. Enable for popups rendered under an
+        // ancestor with transform / filter / perspective / will-change /
+        // contain:layout|paint — there, position: fixed resolves against
+        // that ancestor and the viewport is the wrong clamp edge.
+        clampToContainer: false
       }, options);
 
       let top, left, fixLeft = 0, fixTop = 0, compLeft = 0, compTop = 0;
@@ -35,31 +41,39 @@ export const Position = function(superClass) {
         fixTop = elRect.top;
       }
 
+      // Resolve the clamp rect. Default is the viewport; when the consumer
+      // opts in, prefer the containing-block rect (falling back to viewport
+      // if no qualifying ancestor exists — e.g. the popup is viewport-fixed).
+      const viewport = { top: 0, left: 0, right: window.innerWidth, bottom: window.innerHeight };
+      const clampRect = options.clampToContainer
+        ? (this._containingBlockRect(el) || viewport)
+        : viewport;
+
       const anchorRect = anchor.getBoundingClientRect();
       if (options.valign === 'top') {
         top = anchorRect.top - elRect.height - options.spacing;
 
-        // Move popup down a little bit if there issn't enough room over the anchor.
-        if (top < 0) {
-          compTop = Math.abs(top);
-          top = 0;
+        // Move popup down a little bit if there isn't enough room over the anchor.
+        if (top < clampRect.top) {
+          compTop = clampRect.top - top;
+          top = clampRect.top;
         }
       }
 
       if (options.valign === 'bottom') {
         top = anchorRect.top + anchorRect.height + options.spacing;
 
-        // Move popup up a little bit if there issn't enough room under the anchor.
-        if (top + elRect.height > window.innerHeight) {
-          compTop = top + elRect.height - window.innerHeight;
+        // Move popup up a little bit if there isn't enough room under the anchor.
+        if (top + elRect.height > clampRect.bottom) {
+          compTop = top + elRect.height - clampRect.bottom;
           top -= compTop;
         }
       }
 
-      // Ensure top never goes below 0
-      if (top < 0) {
-        compTop += Math.abs(top);
-        top = 0;
+      // Ensure top never goes above the clamp rect's top edge.
+      if (top < clampRect.top) {
+        compTop += clampRect.top - top;
+        top = clampRect.top;
       }
 
       if (options.halign === 'left') {
@@ -74,18 +88,18 @@ export const Position = function(superClass) {
         left = anchorRect.left + anchorRect.width - elRect.width;
       }
 
-      if (left + elRect.width > window.innerWidth) {
-        compLeft = left + elRect.width - window.innerWidth;
+      if (left + elRect.width > clampRect.right) {
+        compLeft = left + elRect.width - clampRect.right;
         left -= compLeft;
       }
 
-      if (left < 0) {
-        compLeft = Math.abs(left);
-        left = 0;
+      if (left < clampRect.left) {
+        compLeft = clampRect.left - left;
+        left = clampRect.left;
       }
 
-      // Constrain height to viewport - calculate available height from final top position
-      const availableHeight = window.innerHeight - top;
+      // Constrain height to clamp rect — calculate available height from final top.
+      const availableHeight = clampRect.bottom - top;
       if (elRect.height > availableHeight) {
         el.style.maxHeight = availableHeight + 'px';
         el.style.overflowY = 'auto';
@@ -100,5 +114,49 @@ export const Position = function(superClass) {
         compTop: compTop
       };
     }
+
+    /**
+     * Walk up from `el` — across shadow boundaries — to find the nearest
+     * ancestor that establishes a containing block for fixed-positioned
+     * descendants (CSS Transforms / Filter Effects / Containment specs).
+     * Returns that ancestor's viewport rect, or null when `el` is fixed to
+     * the viewport (no qualifying ancestor).
+     */
+    _containingBlockRect(el) {
+      let node = el.parentElement || (el.getRootNode && el.getRootNode().host);
+      while (node) {
+        const cs = getComputedStyle(node);
+        if (cs && this._establishesContainingBlock(cs)) {
+          return node.getBoundingClientRect();
+        }
+        node = node.parentElement || (node.getRootNode && node.getRootNode().host);
+      }
+      return null;
+    }
+
+    /**
+     * Per CSS, an element becomes the containing block for fixed descendants
+     * when it has a non-`none` transform, perspective, filter, backdrop-filter,
+     * a `will-change` listing any of those, or a `contain` value that includes
+     * layout or paint (the `strict` and `content` shorthands qualify).
+     */
+    _establishesContainingBlock(cs) {
+      if (cs.transform && cs.transform !== 'none') return true;
+      if (cs.perspective && cs.perspective !== 'none') return true;
+      if (cs.filter && cs.filter !== 'none') return true;
+      if (cs.backdropFilter && cs.backdropFilter !== 'none') return true;
+      if (cs.webkitBackdropFilter && cs.webkitBackdropFilter !== 'none') return true;
+      const wc = cs.willChange;
+      if (wc && wc !== 'auto' &&
+          (wc.includes('transform') || wc.includes('perspective') || wc.includes('filter'))) {
+        return true;
+      }
+      const c = cs.contain;
+      if (c && c !== 'none' &&
+          (c.includes('layout') || c.includes('paint') || c.includes('strict') || c.includes('content'))) {
+        return true;
+      }
+      return false;
+    }
   };
 }