diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
index b762e8da5286..baf19cfe5590 100644
--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -3735,14 +3735,24 @@ void
 nsWindow::UpdateThemeGeometries(const nsTArray<ThemeGeometry>& aThemeGeometries)
 {
   nsIntRegion clearRegion;
-  for (size_t i = 0; i < aThemeGeometries.Length(); i++) {
-    if (aThemeGeometries[i].mType == nsNativeThemeWin::eThemeGeometryTypeWindowButtons &&
-        nsUXThemeData::CheckForCompositor())
-    {
-      nsIntRect bounds = aThemeGeometries[i].mRect;
-      clearRegion = nsIntRect(bounds.X(), bounds.Y(), bounds.Width(), bounds.Height() - 2.0);
-      clearRegion.Or(clearRegion, nsIntRect(bounds.X() + 1.0, bounds.YMost() - 2.0, bounds.Width() - 1.0, 1.0));
-      clearRegion.Or(clearRegion, nsIntRect(bounds.X() + 2.0, bounds.YMost() - 1.0, bounds.Width() - 3.0, 1.0));
+  if (!HasGlass() || !nsUXThemeData::CheckForCompositor()) {
+    return;
+  }
+  // On Win10, force show the top border:
+  if (IsWin10OrLater() && mCustomNonClient && mSizeMode == nsSizeMode_Normal) {
+    RECT rect;
+    ::GetWindowRect(mWnd, &rect);
+    clearRegion.Or(clearRegion, nsIntRect(0, 0, rect.right - rect.left, 1.0));
+  }
+  if (!IsWin10OrLater()) {
+    for (size_t i = 0; i < aThemeGeometries.Length(); i++) {
+      if (aThemeGeometries[i].mType == nsNativeThemeWin::eThemeGeometryTypeWindowButtons)
+      {
+        nsIntRect bounds = aThemeGeometries[i].mRect;
+        clearRegion.Or(clearRegion, nsIntRect(bounds.X(), bounds.Y(), bounds.Width(), bounds.Height() - 2.0));
+        clearRegion.Or(clearRegion, nsIntRect(bounds.X() + 1.0, bounds.YMost() - 2.0, bounds.Width() - 1.0, 1.0));
+        clearRegion.Or(clearRegion, nsIntRect(bounds.X() + 2.0, bounds.YMost() - 1.0, bounds.Width() - 3.0, 1.0));
+      }
     }
   }
 
@@ -4699,6 +4709,9 @@ nsWindow::ProcessMessage(UINT msg, WPARAM& wParam, LPARAM& lParam,
   LRESULT dwmHitResult;
   if (mCustomNonClient &&
       nsUXThemeData::CheckForCompositor() &&
+      /* We don't do this for win10 glass with a custom titlebar,
+       * in order to avoid the caption buttons breaking. */
+      !(IsWin10OrLater() && HasGlass()) &&
       WinUtils::dwmDwmDefWindowProcPtr(mWnd, msg, wParam, lParam, &dwmHitResult)) {
     *aRetValue = dwmHitResult;
     return true;