Bug 1781046 - Let elements have a last remembered size. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D154322

dom/base/Element.cpp

@@ -482,6 +482,26 @@ void Element::SetShadowRoot(ShadowRoot* aShadowRoot) {
   slots->mShadowRoot = aShadowRoot;
 }
 
+void Element::SetLastRememberedBSize(float aBSize) {
+  ExtendedDOMSlots()->mLastRememberedBSize = Some(aBSize);
+}
+
+void Element::SetLastRememberedISize(float aISize) {
+  ExtendedDOMSlots()->mLastRememberedISize = Some(aISize);
+}
+
+void Element::RemoveLastRememberedBSize() {
+  if (nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots()) {
+    slots->mLastRememberedBSize.reset();
+  }
+}
+
+void Element::RemoveLastRememberedISize() {
+  if (nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots()) {
+    slots->mLastRememberedISize.reset();
+  }
+}
+
 void Element::Blur(mozilla::ErrorResult& aError) {
   if (!ShouldBlur(this)) {
     return;

dom/base/Element.h

@@ -298,6 +298,11 @@ class Element : public FragmentOrElement {
    */
   void SetShadowRoot(ShadowRoot* aShadowRoot);
 
+  void SetLastRememberedBSize(float aBSize);
+  void SetLastRememberedISize(float aISize);
+  void RemoveLastRememberedBSize();
+  void RemoveLastRememberedISize();
+
   /**
    * Make focus on this element.
    */
@@ -1311,6 +1316,21 @@ class Element : public FragmentOrElement {
     return slots ? slots->mShadowRoot.get() : nullptr;
   }
 
+  const Maybe<float> GetLastRememberedBSize() const {
+    const nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
+    return slots ? slots->mLastRememberedBSize : Nothing();
+  }
+  const Maybe<float> GetLastRememberedISize() const {
+    const nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
+    return slots ? slots->mLastRememberedISize : Nothing();
+  }
+  bool HasLastRememberedBSize() const {
+    return GetLastRememberedBSize().isSome();
+  }
+  bool HasLastRememberedISize() const {
+    return GetLastRememberedISize().isSome();
+  }
+
  private:
   // DO NOT USE THIS FUNCTION directly in C++. This function is supposed to be
   // called from JS. Use PresShell::ScrollContentIntoView instead.

dom/base/FragmentOrElement.h

@@ -197,6 +197,13 @@ class FragmentOrElement : public nsIContent {
      * Web components custom element data.
      */
     UniquePtr<CustomElementData> mCustomElementData;
+
+    /**
+     * Last remembered size (in CSS pixels) for the element.
+     * @see {@link https://drafts.csswg.org/css-sizing-4/#last-remembered}
+     */
+    Maybe<float> mLastRememberedBSize;
+    Maybe<float> mLastRememberedISize;
   };
 
   class nsDOMSlots : public nsIContent::nsContentSlots {