Bug 1821618 - Make norolluponanchor="true" work on Wayland moved-to-rect popups. r=stransky

Use the unstransformed anchor rect to get the right original anchor
rect.

Differential Revision: https://phabricator.services.mozilla.com/D172260

layout/xul/nsXULPopupManager.cpp

@@ -368,13 +368,23 @@ bool nsXULPopupManager::RollupInternal(RollupKind aKind,
     nsMenuPopupFrame* popupFrame = item->Frame();
     CSSIntRect anchorRect = [&] {
       if (popupFrame->IsAnchored()) {
-        // Check if the popup has a screen anchor rectangle. If not, get the
+        // Check if the popup has an anchor rectangle set. If not, get the
         // rectangle from the anchor element.
         auto r = popupFrame->GetScreenAnchorRect();
         if (r.x != -1 && r.y != -1) {
+          // Prefer the untransformed anchor rect, so as to account for Wayland
+          // properly. Note we still need to check GetScreenAnchorRect() tho, so
+          // as to detect whether the anchor came from the popup opening call,
+          // or from an element (in which case we want to take the code-path
+          // below)..
+          auto untransformed = popupFrame->GetUntransformedAnchorRect();
+          if (!untransformed.IsEmpty()) {
+            return CSSIntRect::FromAppUnitsRounded(untransformed);
+          }
           return r;
         }
       }
+
       auto* anchor = Element::FromNodeOrNull(popupFrame->GetAnchor());
       if (!anchor) {
         return CSSIntRect();