diff --git a/src/browser/tests/element/position.html b/src/browser/tests/element/position.html
index 6147cb0b4..291bf60f3 100644
--- a/src/browser/tests/element/position.html
+++ b/src/browser/tests/element/position.html
@@ -100,10 +100,13 @@
   testing.expectTrue(zeroWidth.scrollWidth > 0);
 
   // The root containers keep their synthetic size rather than summing
-  // children, so horizontal page-overflow checks stay stable.
+  // children, so horizontal page-overflow checks stay stable. In standards
+  // mode the root element's clientWidth is the viewport (CSSOM View), not
+  // its box; offsetWidth and scrollWidth still report the box.
   testing.expectEqual(document.body.clientWidth, document.body.scrollWidth);
+  testing.expectEqual(window.innerWidth, document.documentElement.clientWidth);
   testing.expectEqual(
-    document.documentElement.clientWidth,
+    document.documentElement.offsetWidth,
     document.documentElement.scrollWidth,
   );
 }
@@ -335,14 +338,16 @@
   testing.expectTrue(panel.scrollHeight > 0);
 
   // The root containers keep their synthetic size rather than summing
-  // children. This is what keeps infinite-scroll triggers reading
-  // `scrollTop + clientHeight >= scrollHeight` on body/documentElement
-  // behaving as they do today.
+  // children. The root element's clientHeight is the viewport (CSSOM View):
+  // jstracker tiles clientWidth/10 cells over clientHeight and calls
+  // elementFromPoint per cell, which the 100M px box turned into ~5M walks.
   testing.expectEqual(document.body.clientHeight, document.body.scrollHeight);
+  testing.expectEqual(window.innerHeight, document.documentElement.clientHeight);
   testing.expectEqual(
-    document.documentElement.clientHeight,
+    document.documentElement.offsetHeight,
     document.documentElement.scrollHeight,
   );
+  testing.expectTrue(document.documentElement.scrollHeight > document.documentElement.clientHeight);
 }
 </script>
 
diff --git a/src/browser/webapi/Element.zig b/src/browser/webapi/Element.zig
index 70b046bca..50a1ad790 100644
--- a/src/browser/webapi/Element.zig
+++ b/src/browser/webapi/Element.zig
@@ -1398,17 +1398,35 @@ pub fn getElementAxis(self: *Element, frame: *Frame, comptime axis: Axis) Axis.S
 // width / height treshold is reached. If the size isn't explicit, we fallback
 // to the content size.
 pub fn getClientWidth(self: *Element, frame: *Frame) f64 {
-    if (!self.checkVisibilityCached(null, frame, .materialize)) {
-        return 0.0;
-    }
-    return self.boxAxis(frame, .width);
+    return self.clientAxis(frame, .width);
 }
 
 pub fn getClientHeight(self: *Element, frame: *Frame) f64 {
+    return self.clientAxis(frame, .height);
+}
+
+fn clientAxis(self: *Element, frame: *Frame, comptime axis: Axis) f64 {
     if (!self.checkVisibilityCached(null, frame, .materialize)) {
         return 0.0;
     }
-    return self.boxAxis(frame, .height);
+    return self.viewportAxis(frame, axis) orelse self.boxAxis(frame, axis);
+}
+
+fn viewportAxis(self: *Element, frame: *Frame, comptime axis: Axis) ?f64 {
+    const tag = self.getTag();
+    if (tag != .html and tag != .body) {
+        return null;
+    }
+    const doc = self.asNode().ownerDocument(frame) orelse frame.document;
+    if ((tag == .body) != doc.isQuirksMode()) {
+        return null;
+    }
+    // In quicks mode, the root element (the body) reports the viewport for
+    // clientWidth and clientHeight rather than its own MASSIVE box. This
+    // fixes jstracker's uiContourMap which attempts to tile the clientHeight
+    // of the body. (https://github.com/lightpanda-io/browser/issues/3251)
+    const viewport = frame._page.getViewport();
+    return @floatFromInt(if (axis == .width) viewport.width else viewport.height);
 }
 
 // Caller must have made sure self is visible.
@@ -1581,12 +1599,20 @@ fn contentAxis(self: *Element, frame: *Frame, comptime axis: Axis) f64 {
     return total;
 }
 
+// Unlike clientHeight, the root's offsetHeight is its box (the document
+// extent), so it stays on the synthetic root default.
 pub fn getOffsetHeight(self: *Element, frame: *Frame) f64 {
-    return self.getClientHeight(frame);
+    if (!self.checkVisibilityCached(null, frame, .materialize)) {
+        return 0.0;
+    }
+    return self.boxAxis(frame, .height);
 }
 
 pub fn getOffsetWidth(self: *Element, frame: *Frame) f64 {
-    return self.getClientWidth(frame);
+    if (!self.checkVisibilityCached(null, frame, .materialize)) {
+        return 0.0;
+    }
+    return self.boxAxis(frame, .width);
 }
 
 pub fn getOffsetTop(self: *Element, frame: *Frame) f64 {