diff --git a/toolkit/content/widgets/dialog.js b/toolkit/content/widgets/dialog.js
index 862f891154e9..6a32efc7f6c2 100644
--- a/toolkit/content/widgets/dialog.js
+++ b/toolkit/content/widgets/dialog.js
@@ -196,6 +196,8 @@
       return this.shadowRoot.querySelector(".dialog-button-box");
     }
 
+    // NOTE(emilio): This has to match AppWindow::IntrinsicallySizeShell, to
+    // prevent flickering, see bug 1799394.
     _sizeToPreferredSize() {
       const docEl = document.documentElement;
       const prefWidth = (() => {
diff --git a/xpcom/ds/StaticAtoms.py b/xpcom/ds/StaticAtoms.py
index b680e87ea4fd..af190caa7682 100644
--- a/xpcom/ds/StaticAtoms.py
+++ b/xpcom/ds/StaticAtoms.py
@@ -1012,9 +1012,10 @@ STATIC_ATOMS = [
     Atom("prefersReducedMotion", "prefers-reduced-motion"),
     Atom("prefersColorScheme", "prefers-color-scheme"),
     Atom("prefersContrast", "prefers-contrast"),
+    Atom("prefix", "prefix"),
+    Atom("prefwidth", "prefwidth"),
     Atom("dynamicRange", "dynamic-range"),
     Atom("videoDynamicRange", "video-dynamic-range"),
-    Atom("prefix", "prefix"),
     Atom("preload", "preload"),
     Atom("preserve", "preserve"),
     Atom("preserveSpace", "preserve-space"),
diff --git a/xpfe/appshell/AppWindow.cpp b/xpfe/appshell/AppWindow.cpp
index 116c23d01d58..4e1331fdb443 100644
--- a/xpfe/appshell/AppWindow.cpp
+++ b/xpfe/appshell/AppWindow.cpp
@@ -2598,6 +2598,55 @@ void AppWindow::LoadPersistentWindowState() {
   loadValue(nsGkAtoms::sizemode);
 }
 
+void AppWindow::IntrinsicallySizeShell(const CSSIntSize& aWindowDiff,
+                                       int32_t& aSpecWidth,
+                                       int32_t& aSpecHeight) {
+  nsCOMPtr<nsIContentViewer> cv;
+  mDocShell->GetContentViewer(getter_AddRefs(cv));
+  if (!cv) {
+    return;
+  }
+  RefPtr<nsDocShell> docShell = mDocShell;
+  nsCOMPtr<nsIDocShellTreeOwner> treeOwner;
+  docShell->GetTreeOwner(getter_AddRefs(treeOwner));
+  if (!treeOwner) {
+    return;
+  }
+
+  CSSIntCoord maxWidth = 0;
+  CSSIntCoord maxHeight = 0;
+  CSSIntCoord prefWidth = 0;
+  if (RefPtr element = GetWindowDOMElement()) {
+    nsAutoString prefWidthAttr;
+    if (element->GetAttr(nsGkAtoms::prefwidth, prefWidthAttr)) {
+      // TODO: Make this more generic perhaps?
+      if (prefWidthAttr.EqualsLiteral("min-width")) {
+        if (auto* f = element->GetPrimaryFrame(FlushType::Frames)) {
+          const auto& coord = f->StylePosition()->mMinWidth;
+          if (coord.ConvertsToLength()) {
+            prefWidth = CSSPixel::FromAppUnitsRounded(coord.ToLength());
+          }
+        }
+      }
+    }
+  }
+
+  Maybe<CSSIntSize> size = cv->GetContentSize(maxWidth, maxHeight, prefWidth);
+  if (!size) {
+    return;
+  }
+  nsPresContext* pc = cv->GetPresContext();
+  MOZ_ASSERT(pc, "Should have pres context");
+
+  int32_t width = pc->CSSPixelsToDevPixels(size->width);
+  int32_t height = pc->CSSPixelsToDevPixels(size->height);
+  treeOwner->SizeShellTo(docShell, width, height);
+
+  // Update specified size for the final LoadPositionFromXUL call.
+  aSpecWidth = size->width + aWindowDiff.width;
+  aSpecHeight = size->height + aWindowDiff.height;
+}
+
 void AppWindow::SizeShell() {
   AutoRestore<bool> sizingShellFromXUL(mSizingShellFromXUL);
   mSizingShellFromXUL = true;
@@ -2610,7 +2659,7 @@ void AppWindow::SizeShell() {
     windowElement->GetAttr(nsGkAtoms::windowtype, windowType);
   }
 
-  CSSIntSize windowDiff = GetOuterToInnerSizeDifferenceInCSSPixels(
+  const CSSIntSize windowDiff = GetOuterToInnerSizeDifferenceInCSSPixels(
       mWindow, UnscaledDevicePixelsPerCSSPixel());
 
   // If we're using fingerprint resistance, we're going to resize the window
@@ -2652,28 +2701,9 @@ void AppWindow::SizeShell() {
     SetSpecifiedSize(specWidth, specHeight);
   }
 
+  // If LoadSizeFromXUL set the size, mIntrinsicallySized will be false.
   if (mIntrinsicallySized) {
-    // (if LoadSizeFromXUL set the size, mIntrinsicallySized will be false)
-    nsCOMPtr<nsIContentViewer> cv;
-    mDocShell->GetContentViewer(getter_AddRefs(cv));
-    if (cv) {
-      RefPtr<nsDocShell> docShell = mDocShell;
-      nsCOMPtr<nsIDocShellTreeOwner> treeOwner;
-      docShell->GetTreeOwner(getter_AddRefs(treeOwner));
-      if (treeOwner) {
-        if (Maybe<CSSIntSize> size = cv->GetContentSize()) {
-          nsPresContext* pc = cv->GetPresContext();
-          MOZ_ASSERT(pc, "Should have pres context");
-          int32_t width = pc->CSSPixelsToDevPixels(size->width);
-          int32_t height = pc->CSSPixelsToDevPixels(size->height);
-
-          treeOwner->SizeShellTo(docShell, width, height);
-          // Update specified size for the final LoadPositionFromXUL call.
-          specWidth = width + windowDiff.width;
-          specHeight = height + windowDiff.height;
-        }
-      }
-    }
+    IntrinsicallySizeShell(windowDiff, specWidth, specHeight);
   }
 
   // Now that we have set the window's final size, we can re-do its
diff --git a/xpfe/appshell/AppWindow.h b/xpfe/appshell/AppWindow.h
index 9f4eecb9294a..b0d30f3499d5 100644
--- a/xpfe/appshell/AppWindow.h
+++ b/xpfe/appshell/AppWindow.h
@@ -374,6 +374,10 @@ class AppWindow final : public nsIBaseWindow,
   WidgetListenerDelegate mWidgetListenerDelegate;
 
  private:
+  MOZ_CAN_RUN_SCRIPT void IntrinsicallySizeShell(const CSSIntSize& aWindowDiff,
+                                                 int32_t& aSpecWidth,
+                                                 int32_t& aSpecHeight);
+
   // GetPrimaryBrowserParentSize is called from xpidl methods and we don't have
   // a good way to annotate those with MOZ_CAN_RUN_SCRIPT yet.  It takes no
   // refcounted args other than "this", and the "this" uses seem ok.