Bug 1920646 - part 1: Make HTMLEditor paste/drop things as plaintex…

…t when `contenteditable=plaintext-only` r=m_kato

Chrome sets `beforeinput.data` instead of `beforeinput.dataTransfer`, but
Input Events Level 2 spec defines that browsers should set `dataTransfer` when
**contenteditable** [1].  Therefore, the new WPT expects `dataTransfer`.

However, it's unclear that the `dataTransfer` should have `text/html` or only
`text/plain`.  From web apps point of view, `text/html` data may make them
serialize the rich text format to plaintext without any dependencies of browsers
and OS.  On the other hand, they cannot distinguish whether the user tries to
paste with or without formatting when `contenteditable=true`.  Therefore, I
filed a spec issue for this.  We need to be back later about this issue.

1. https://w3c.github.io/input-events/#overview
2. w3c/input-events#162

Differential Revision: https://phabricator.services.mozilla.com/D223908

editor/libeditor/HTMLEditor.h

@@ -3142,9 +3142,10 @@ class HTMLEditor final : public EditorBase,
    *
    * @param aClipboardType      nsIClipboard::kGlobalClipboard or
    *                            nsIClipboard::kSelectionClipboard.
+   * @param aEditingHost        The editing host.
    */
-  MOZ_CAN_RUN_SCRIPT nsresult
-  PasteInternal(nsIClipboard::ClipboardType aClipboardType);
+  MOZ_CAN_RUN_SCRIPT nsresult PasteInternal(
+      nsIClipboard::ClipboardType aClipboardType, const Element& aEditingHost);
 
   [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
   InsertWithQuotationsAsSubAction(const nsAString& aQuotedText) final;
@@ -3198,8 +3199,8 @@ class HTMLEditor final : public EditorBase,
    * with <span _moz_quote="true">, and each chunk not starting with ">" is
    * inserted as normal text.
    */
-  MOZ_CAN_RUN_SCRIPT nsresult
-  InsertTextWithQuotationsInternal(const nsAString& aStringToInsert);
+  MOZ_CAN_RUN_SCRIPT nsresult InsertTextWithQuotationsInternal(
+      const nsAString& aStringToInsert, const Element& aEditingHost);
 
   /**
    * ReplaceContainerWithTransactionInternal() is implementation of
@@ -3722,20 +3723,23 @@ class HTMLEditor final : public EditorBase,
   [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult SetSelectionAtDocumentStart();
 
   // Methods for handling plaintext quotations
-  MOZ_CAN_RUN_SCRIPT nsresult
-  PasteAsPlaintextQuotation(nsIClipboard::ClipboardType aSelectionType);
+  MOZ_CAN_RUN_SCRIPT nsresult PasteAsPlaintextQuotation(
+      nsIClipboard::ClipboardType aSelectionType, const Element& aEditingHost);
 
+  enum class AddCites { No, Yes };
   /**
    * Insert a string as quoted text, replacing the selected text (if any).
    * @param aQuotedText     The string to insert.
    * @param aAddCites       Whether to prepend extra ">" to each line
    *                        (usually true, unless those characters
    *                        have already been added.)
+   * @param aEditingHost    The editing host.
    * @return aNodeInserted  The node spanning the insertion, if applicable.
    *                        If aAddCites is false, this will be null.
    */
   [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult InsertAsPlaintextQuotation(
-      const nsAString& aQuotedText, bool aAddCites, nsINode** aNodeInserted);
+      const nsAString& aQuotedText, AddCites aAddCites,
+      const Element& aEditingHost, nsINode** aNodeInserted = nullptr);
 
   /**
    * InsertObject() inserts given object at aPointToInsert.
@@ -3749,7 +3753,8 @@ class HTMLEditor final : public EditorBase,
       DeleteSelectedContent aDeleteSelectedContent);
 
   class HTMLTransferablePreparer;
-  nsresult PrepareHTMLTransferable(nsITransferable** aTransferable) const;
+  nsresult PrepareHTMLTransferable(nsITransferable** aTransferable,
+                                   const Element* aEditingHost) const;
 
   enum class HavePrivateHTMLFlavor { No, Yes };
   MOZ_CAN_RUN_SCRIPT nsresult InsertFromTransferableAtSelection(
@@ -3765,7 +3770,8 @@ class HTMLEditor final : public EditorBase,
   MOZ_CAN_RUN_SCRIPT nsresult InsertFromDataTransfer(
       const dom::DataTransfer* aDataTransfer, uint32_t aIndex,
       nsIPrincipal* aSourcePrincipal, const EditorDOMPoint& aDroppedAt,
-      DeleteSelectedContent aDeleteSelectedContent);
+      DeleteSelectedContent aDeleteSelectedContent,
+      const Element& aEditingHost);
 
   static HavePrivateHTMLFlavor ClipboardHasPrivateHTMLFlavor(
       nsIClipboard* clipboard);