diff --git a/devtools/client/framework/toolbox-window.xhtml b/devtools/client/framework/toolbox-window.xhtml
index 09960681b763..a3ecc39ae41d 100644
--- a/devtools/client/framework/toolbox-window.xhtml
+++ b/devtools/client/framework/toolbox-window.xhtml
@@ -5,13 +5,19 @@
 
 <?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
 
-<!-- minwidth=50 is sum width of chevron and meatball menu button. -->
+<!--
+  min-width: min-content makes us have the min size of our
+  browser element, which is set explicitly via css in
+  toolbox-host-manager.js. It could be set on the window
+  instead and then it wouldn't be needed.
+-->
 <window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
         id="devtools-toolbox-window"
         macanimationtype="document"
         fullscreenbutton="true"
         windowtype="devtools:toolbox"
         width="900" height="320"
+        style="min-width: min-content"
         persist="screenX screenY width height sizemode">
   <tooltip id="aHTMLTooltip" page="true"/>
 </window>
diff --git a/gfx/src/nsSize.h b/gfx/src/nsSize.h
index b4125e603585..823630ec9f76 100644
--- a/gfx/src/nsSize.h
+++ b/gfx/src/nsSize.h
@@ -26,6 +26,10 @@ struct nsSize : public mozilla::gfx::BaseSize<nscoord, nsSize> {
       float aXScale, float aYScale, nscoord aAppUnitsPerPixel) const;
   inline mozilla::gfx::IntSize ToNearestPixels(nscoord aAppUnitsPerPixel) const;
 
+  inline mozilla::gfx::IntSize ScaleToOutsidePixels(
+      float aXScale, float aYScale, nscoord aAppUnitsPerPixel) const;
+  inline mozilla::gfx::IntSize ToOutsidePixels(nscoord aAppUnitsPerPixel) const;
+
   /**
    * Return this size scaled to a different appunits per pixel (APP) ratio.
    * @param aFromAPP the APP to scale from
@@ -44,11 +48,24 @@ inline mozilla::gfx::IntSize nsSize::ScaleToNearestPixels(
                      aYScale));
 }
 
+inline mozilla::gfx::IntSize nsSize::ScaleToOutsidePixels(
+    float aXScale, float aYScale, nscoord aAppUnitsPerPixel) const {
+  return mozilla::gfx::IntSize(
+      NSToIntCeil(NSAppUnitsToDoublePixels(width, aAppUnitsPerPixel) * aXScale),
+      NSToIntCeil(NSAppUnitsToDoublePixels(height, aAppUnitsPerPixel) *
+                  aYScale));
+}
+
 inline mozilla::gfx::IntSize nsSize::ToNearestPixels(
     nscoord aAppUnitsPerPixel) const {
   return ScaleToNearestPixels(1.0f, 1.0f, aAppUnitsPerPixel);
 }
 
+inline mozilla::gfx::IntSize nsSize::ToOutsidePixels(
+    nscoord aAppUnitsPerPixel) const {
+  return ScaleToOutsidePixels(1.0f, 1.0f, aAppUnitsPerPixel);
+}
+
 inline nsSize nsSize::ScaleToOtherAppUnits(int32_t aFromAPP,
                                            int32_t aToAPP) const {
   if (aFromAPP != aToAPP) {
diff --git a/layout/base/Units.h b/layout/base/Units.h
index 2240f65c6894..0aa3a5aab18a 100644
--- a/layout/base/Units.h
+++ b/layout/base/Units.h
@@ -429,6 +429,12 @@ struct LayoutDevicePixel {
         aRect.ToOutsidePixels(aAppUnitsPerDevPixel));
   }
 
+  static LayoutDeviceIntSize FromAppUnitsToOutside(
+      const nsSize& aSize, nscoord aAppUnitsPerDevPixel) {
+    return LayoutDeviceIntSize::FromUnknownSize(
+        aSize.ToOutsidePixels(aAppUnitsPerDevPixel));
+  }
+
   static LayoutDeviceIntSize FromAppUnitsRounded(const nsSize& aSize,
                                                  nscoord aAppUnitsPerDevPixel) {
     return LayoutDeviceIntSize(
diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index e82300814b48..cb3e53c9347c 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -3136,8 +3136,10 @@ nsresult nsDocumentViewer::GetContentSizeInternal(int32_t* aWidth,
 
   // Ceil instead of rounding here, so we can actually guarantee showing all the
   // content.
-  *aWidth = std::ceil(presContext->AppUnitsToFloatDevPixels(shellArea.width));
-  *aHeight = std::ceil(presContext->AppUnitsToFloatDevPixels(shellArea.height));
+  auto devOuterSize = LayoutDeviceIntSize::FromAppUnitsToOutside(
+      shellArea, presContext->AppUnitsPerDevPixel());
+  *aWidth = devOuterSize.width;
+  *aHeight = devOuterSize.height;
 
   return NS_OK;
 }
diff --git a/layout/generic/nsContainerFrame.cpp b/layout/generic/nsContainerFrame.cpp
index 549cef84ff16..154d8f6841ca 100644
--- a/layout/generic/nsContainerFrame.cpp
+++ b/layout/generic/nsContainerFrame.cpp
@@ -551,18 +551,22 @@ static bool IsTopLevelWidget(nsIWidget* aWidget) {
 void nsContainerFrame::SyncWindowProperties(nsPresContext* aPresContext,
                                             nsIFrame* aFrame, nsView* aView,
                                             gfxContext* aRC, uint32_t aFlags) {
-#ifdef MOZ_XUL
-  if (!aView || !nsCSSRendering::IsCanvasFrame(aFrame) || !aView->HasWidget())
+  if (!aView || !nsCSSRendering::IsCanvasFrame(aFrame) || !aView->HasWidget()) {
     return;
+  }
 
   nsCOMPtr<nsIWidget> windowWidget =
       GetPresContextContainerWidget(aPresContext);
-  if (!windowWidget || !IsTopLevelWidget(windowWidget)) return;
+  if (!windowWidget || !IsTopLevelWidget(windowWidget)) {
+    return;
+  }
 
   nsViewManager* vm = aView->GetViewManager();
   nsView* rootView = vm->GetRootView();
 
-  if (aView != rootView) return;
+  if (aView != rootView) {
+    return;
+  }
 
   Element* rootElement = aPresContext->Document()->GetRootElement();
   if (!rootElement) {
@@ -571,7 +575,9 @@ void nsContainerFrame::SyncWindowProperties(nsPresContext* aPresContext,
 
   nsIFrame* rootFrame =
       aPresContext->PresShell()->FrameConstructor()->GetRootElementStyleFrame();
-  if (!rootFrame) return;
+  if (!rootFrame) {
+    return;
+  }
 
   if (aFlags & SET_ASYNC) {
     aView->SetNeedsWindowPropertiesSync();
@@ -600,7 +606,9 @@ void nsContainerFrame::SyncWindowProperties(nsPresContext* aPresContext,
     windowWidget->SetWindowShadowStyle(shadow);
   }
 
-  if (!aRC) return;
+  if (!aRC) {
+    return;
+  }
 
   if (!weak.IsAlive()) {
     return;
@@ -608,36 +616,49 @@ void nsContainerFrame::SyncWindowProperties(nsPresContext* aPresContext,
 
   nsSize minSize(0, 0);
   nsSize maxSize(NS_UNCONSTRAINEDSIZE, NS_UNCONSTRAINEDSIZE);
-  if (rootElement->IsXULElement()) {
-    nsBoxLayoutState aState(aPresContext, aRC);
-    minSize = rootFrame->GetXULMinSize(aState);
-    maxSize = rootFrame->GetXULMaxSize(aState);
-  } else {
-    auto* pos = rootFrame->StylePosition();
-    if (pos->mMinWidth.ConvertsToLength()) {
-      minSize.width = pos->mMinWidth.ToLength();
-    }
-    if (pos->mMinHeight.ConvertsToLength()) {
-      minSize.height = pos->mMinHeight.ToLength();
-    }
-    if (pos->mMaxWidth.ConvertsToLength()) {
-      maxSize.width = pos->mMaxWidth.ToLength();
-    }
-    if (pos->mMaxHeight.ConvertsToLength()) {
-      maxSize.height = pos->mMaxHeight.ToLength();
+
+  const auto& pos = *rootFrame->StylePosition();
+#ifdef MOZ_XUL
+  if (rootFrame->IsXULBoxFrame()) {
+    nsBoxLayoutState xulState(aPresContext, aRC);
+    minSize = rootFrame->GetXULMinSize(xulState);
+    const bool isMinContent =
+        pos.mMinWidth.IsExtremumLength() &&
+        pos.mMinWidth.AsExtremumLength() == StyleExtremumLength::MinContent;
+    // By default we behave with min-width: max-content, to ensure that all
+    // content is shown (i.e., the GetXULPrefSize call below).
+    //
+    // It can be overridden to min-content (i.e., leave GetXULMinSize()) and
+    // a fixed width (handled below).
+    if (!isMinContent && !pos.mMinWidth.ConvertsToLength()) {
+      minSize.width = rootFrame->GetXULPrefSize(xulState).width;
     }
+    maxSize = rootFrame->GetXULMaxSize(xulState);
   }
-  SetSizeConstraints(aPresContext, windowWidget, minSize, maxSize);
 #endif
+
+  if (pos.mMinWidth.ConvertsToLength()) {
+    minSize.width = pos.mMinWidth.ToLength();
+  }
+  if (pos.mMinHeight.ConvertsToLength()) {
+    minSize.height = pos.mMinHeight.ToLength();
+  }
+  if (pos.mMaxWidth.ConvertsToLength()) {
+    maxSize.width = pos.mMaxWidth.ToLength();
+  }
+  if (pos.mMaxHeight.ConvertsToLength()) {
+    maxSize.height = pos.mMaxHeight.ToLength();
+  }
+
+  SetSizeConstraints(aPresContext, windowWidget, minSize, maxSize);
 }
 
 void nsContainerFrame::SetSizeConstraints(nsPresContext* aPresContext,
                                           nsIWidget* aWidget,
                                           const nsSize& aMinSize,
                                           const nsSize& aMaxSize) {
-  LayoutDeviceIntSize devMinSize(
-      aPresContext->AppUnitsToDevPixels(aMinSize.width),
-      aPresContext->AppUnitsToDevPixels(aMinSize.height));
+  auto devMinSize = LayoutDeviceIntSize::FromAppUnitsToOutside(
+      aMinSize, aPresContext->AppUnitsPerDevPixel());
   LayoutDeviceIntSize devMaxSize(
       aMaxSize.width == NS_UNCONSTRAINEDSIZE
           ? NS_MAXSIZE
@@ -647,9 +668,8 @@ void nsContainerFrame::SetSizeConstraints(nsPresContext* aPresContext,
           : aPresContext->AppUnitsToDevPixels(aMaxSize.height));
 
   // MinSize has a priority over MaxSize
-  if (devMinSize.width > devMaxSize.width) devMaxSize.width = devMinSize.width;
-  if (devMinSize.height > devMaxSize.height)
-    devMaxSize.height = devMinSize.height;
+  devMaxSize.width = std::max(devMinSize.width, devMaxSize.width);
+  devMaxSize.height = std::max(devMinSize.height, devMaxSize.height);
 
   widget::SizeConstraints constraints(devMinSize, devMaxSize);
 
diff --git a/toolkit/content/tests/chrome/window_screenPosSize.xhtml b/toolkit/content/tests/chrome/window_screenPosSize.xhtml
index accc10d8f10b..cfb9f38343b1 100644
--- a/toolkit/content/tests/chrome/window_screenPosSize.xhtml
+++ b/toolkit/content/tests/chrome/window_screenPosSize.xhtml
@@ -8,6 +8,7 @@
         screenY="80"
         height="300"
         width="300"
+        style="min-width: 0"
         persist="screenX screenY height width">
 
 <body xmlns="http://www.w3.org/1999/xhtml">