Bug 1840856: Fixed linefeed/linebreak issues for contenteditable. r=m…

…asayuki,dom-core

This patch is related to Bug 1840822, which fixes some issues
regarding additional invisible linebreaks needed at the end of
contenteditables.
This patch in particular fixes the case where the user
presses enter at the end of a pre-formatted
contenteditable, e.g.:
```
<span contenteditable style="display:block;white-space:pre-line">[]</span>
```
Prior to this patch, only one `<br>` was added,
the invisible padding linebreak was missing.
With this patch applied, the behaviour should be as expected:
```
<span contenteditable style="display:block;white-space:pre-line">\n<br></span>
```

Differential Revision: https://phabricator.services.mozilla.com/D190217

editor/libeditor/HTMLEditSubActionHandler.cpp

@@ -1871,8 +1871,9 @@ HTMLEditor::InsertParagraphSeparatorAsSubAction(const Element& aEditingHost) {
           return aDefaultParagraphSeparator == ParagraphSeparator::br ||
                  !HTMLEditUtils::CanElementContainParagraph(
                      *aEditableBlockElement) ||
-                 HTMLEditUtils::ShouldInsertLinefeedCharacter(
-                     aCandidatePointToSplit, aEditingHost);
+                 (HTMLEditUtils::ShouldInsertLinefeedCharacter(
+                      aCandidatePointToSplit, aEditingHost) &&
+                  HTMLEditUtils::IsDisplayOutsideInline(aEditingHost));
         }
 
         // If the nearest block parent is a single-line container declared in

editor/libeditor/HTMLEditUtils.cpp

@@ -1041,11 +1041,9 @@ bool HTMLEditUtils::ShouldInsertLinefeedCharacter(
           BlockInlineCheck::UseComputedDisplayOutsideStyle);
 
   // If and only if the nearest block is the editing host or its parent,
-  // and the outer display value of the editing host is inline, and new
-  // line character is preformatted, we should insert a linefeed.
+  // and new line character is preformatted, we should insert a linefeed.
   return (!closestEditableBlockElement ||
           closestEditableBlockElement == &aEditingHost) &&
-         HTMLEditUtils::IsDisplayOutsideInline(aEditingHost) &&
          EditorUtils::IsNewLinePreformatted(
              *aPointToInsert.ContainerAs<nsIContent>());
 }