diff --git a/layout/base/PresShell.h b/layout/base/PresShell.h
index 1bdebe0b37c0..ac8c20c9b9b5 100644
--- a/layout/base/PresShell.h
+++ b/layout/base/PresShell.h
@@ -1593,13 +1593,34 @@ class PresShell final : public nsStubDocumentObserver,
   /**
    * Tells the presshell to scroll again to the last anchor scrolled to by
    * GoToAnchor, if any. This scroll only happens if the scroll
-   * position has not changed since the last GoToAnchor. This is called
-   * by nsDocumentViewer::LoadComplete. This clears the last anchor
-   * scrolled to by GoToAnchor (we don't want to keep it alive if it's
-   * removed from the DOM), so don't call this more than once.
+   * position has not changed since the last GoToAnchor (modulo scroll anchoring
+   * adjustments). This is called by nsDocumentViewer::LoadComplete. This clears
+   * the last anchor scrolled to by GoToAnchor (we don't want to keep it alive
+   * if it's removed from the DOM), so don't call this more than once.
    */
   MOZ_CAN_RUN_SCRIPT nsresult ScrollToAnchor();
 
+  /**
+   * When scroll anchoring adjusts positions in the root frame during page load,
+   * it may move our scroll position in the root frame.
+   *
+   * While that's generally desirable, when scrolling to an anchor via an id-ref
+   * we have a more direct target. If the id-ref points to something that cannot
+   * be selected as a scroll anchor container (like an image or an inline), we
+   * may select a node following it as a scroll anchor, and if then stuff is
+   * inserted on top, we may end up moving the id-ref element offscreen to the
+   * top inadvertently.
+   *
+   * On page load, the document viewer will call ScrollToAnchor(), and will only
+   * scroll to the anchor again if the scroll position is not changed. We don't
+   * want scroll anchoring adjustments to prevent this, so account for them.
+   */
+  void RootScrollFrameAdjusted(nscoord aYAdjustment) {
+    if (mLastAnchorScrolledTo) {
+      mLastAnchorScrollPositionY += aYAdjustment;
+    }
+  }
+
   /**
    * Scrolls the view of the document so that the primary frame of the content
    * is displayed in the window. Layout is flushed before scrolling.
diff --git a/layout/generic/ScrollAnchorContainer.cpp b/layout/generic/ScrollAnchorContainer.cpp
index 7efec2b63a12..b95ac7793798 100644
--- a/layout/generic/ScrollAnchorContainer.cpp
+++ b/layout/generic/ScrollAnchorContainer.cpp
@@ -378,8 +378,10 @@ void ScrollAnchorContainer::ApplyAdjustments() {
   mApplyingAnchorAdjustment = false;
 
   nsPresContext* pc = Frame()->PresContext();
-  Document* doc = pc->Document();
-  doc->UpdateForScrollAnchorAdjustment(logicalAdjustment);
+  if (mScrollFrame->mIsRoot) {
+    pc->PresShell()->RootScrollFrameAdjusted(physicalAdjustment.y);
+  }
+  pc->Document()->UpdateForScrollAnchorAdjustment(logicalAdjustment);
 
   // The anchor position may not be in the same relative position after
   // adjustment. Update ourselves so we have consistent state.
diff --git a/layout/generic/test/file_SlowTallImage.sjs b/layout/generic/test/file_SlowTallImage.sjs
new file mode 100644
index 000000000000..6297cf23e073
--- /dev/null
+++ b/layout/generic/test/file_SlowTallImage.sjs
@@ -0,0 +1,20 @@
+"use strict";
+
+/* eslint-disable-next-line mozilla/use-chromeutils-import */
+let {setTimeout} = Cu.import("resource://gre/modules/Timer.jsm", {});
+
+// A tall 1x1000 black png.
+const IMG_BYTES = atob(
+  "iVBORw0KGgoAAAANSUhEUgAAAAEAAAPoAQMAAAAleAYdAAAABlBMVEUAAAD///+l2Z/dAAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAF0lEQVQ4jWNgGAWjYBSMglEwCkbBUAcAB9AAASBs/t4AAAAASUVORK5CYII="
+);
+
+// Cargo-culted from file_SlowImage.sjs
+function handleRequest(request, response) {
+  response.processAsync();
+  response.setHeader("Content-Type", "image/png");
+  let delay = request.queryString.indexOf("slow") >= 0 ? 600 : 200;
+  setTimeout(function() {
+    response.write(IMG_BYTES);
+    response.finish();
+  }, delay);
+}
diff --git a/layout/generic/test/file_bug1566783.html b/layout/generic/test/file_bug1566783.html
new file mode 100644
index 000000000000..77c5658cde4c
--- /dev/null
+++ b/layout/generic/test/file_bug1566783.html
@@ -0,0 +1,24 @@
+<!doctype html>
+<style>
+.spacer { height: 200vh; }
+</style>
+<script>
+  function loadFailed() {
+    parent.ok(false, "Image load should not fail");
+  }
+</script>
+<div class="spacer"></div>
+<img id="fast" src="file_SlowTallImage.sjs?fast" onerror="loadFailed()">
+<div class="spacer"></div>
+<img id="slow" src="file_SlowTallImage.sjs?slow" onerror="loadFailed()">
+<div class="spacer"></div>
+<script>
+onload = function() {
+  setTimeout(function() {
+    let rect = document.getElementById("slow").getBoundingClientRect();
+    parent.is(rect.height, 1000, "#slow should take space");
+    parent.is(rect.top, 0, "#slow should be at the top of the viewport");
+    parent.SimpleTest.finish();
+  }, 0);
+}
+</script>
diff --git a/layout/generic/test/mochitest.ini b/layout/generic/test/mochitest.ini
index a9acda1a7d23..ee81e874acfc 100644
--- a/layout/generic/test/mochitest.ini
+++ b/layout/generic/test/mochitest.ini
@@ -16,6 +16,7 @@ support-files =
   file_LoadingImageReference.png
   file_SlowImage.sjs
   file_SlowPage.sjs
+  file_SlowTallImage.sjs
   bug1174521.html
   !/gfx/layers/apz/test/mochitest/apz_test_utils.js
 
@@ -100,6 +101,8 @@ support-files = bug633762_iframe.html
 support-files = file_bug1307853.html
 [test_bug1408607.html]
 [test_bug1499961.html]
+[test_bug1566783.html]
+support-files = file_bug1566783.html
 [test_contained_plugin_transplant.html]
 skip-if = os=='win'
 [test_dynamic_reflow_root_disallowal.html]
diff --git a/layout/generic/test/test_bug1566783.html b/layout/generic/test/test_bug1566783.html
new file mode 100644
index 000000000000..4632b0718b15
--- /dev/null
+++ b/layout/generic/test/test_bug1566783.html
@@ -0,0 +1,8 @@
+<!doctype html>
+<title>Test for scroll anchoring adjustments during onload</title>
+<script src="/tests/SimpleTest/SimpleTest.js"></script>
+<script>
+  SimpleTest.waitForExplicitFinish();
+</script>
+<link rel="stylesheet" href="/tests/SimpleTest/test.css"/>
+<iframe width="300" height="300" src="file_bug1566783.html#slow"></iframe>