Make HTMLEditor::HandleInsertParagraphInParagraph() call WhiteSpaceVisibilityKeeper::PrepareToSplitBlockElement() before splitting a text node

It does the following things when caret is collapsed in a text node in a `<p>`
or `<div>` element.

1. Split the text node containing caret to insert `<br>` element
2. Insert `<br>` element after it
3. Split ancestor elements which inclusive descendants of the `<p>` or `<div>`
4. Delete the `<br>` element if unnecessary from the left paragraph

#3 and #4 are performed by `HTMLEditor::SplitParagraph()` and it calls
`WhiteSpaceVisibilityKeeper::PrepareToSplitBlockElement()` correctly before
splitting the block.  However, in the case (caret is at middle of a text node),
the text has already been split to 2 nodes because of #1.  Therefore, it fails
to handle to keep the white-space visibility.

So that I believe that the root cause of this bug is, the method does much
complicated things which are required, and doing the redundant things will
eat memory space due to undo transactions.  However, for now, I'd like to fix
this with a simple patch which just call the preparation method before splitting
the text node because I'd like to uplift this if it'd be approved (Note that
this is not a recent regression, the root cause was created by bug 92686 which
was fixed in 17 years ago:
<https://searchfox.org/mozilla-central/commit/2e66280faef73e9be218e00758d4eb738395ac83>,
but must be annoying bug for users who see this frequently).

The new WPTs are pass in Chrome.

Differential Revision: https://phabricator.services.mozilla.com/D130950

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1740416
gecko-commit: 73567f6c2bcfa078836a36760498bb11747561dd
gecko-reviewers: m_kato, smaug

Author: masayuki-nakano

editing/data/insertparagraph.js

@@ -1990,4 +1990,109 @@ var browserTests = [
      "<dl contenteditable=\"false\"><dd contenteditable=\"\"><div>ab</div><div>cd<br></div>/dd></dl>"],
     [true,true],
     {"insertparagraph":[false,false,"",false,false,""]}],
+
+// The first white-space of the second paragraph must be &nbsp; for making it
+// visible, but the other things do not matter for the following tests.
+["<div>a[] b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    "<div>a</div><div>&nbsp;b</div>",
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+// And if the first paragraph ends with white-space, the text node should be
+// followed by <br> element.
+["<div>a []b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a <br></div><div>b</div>",
+     "<div>a&nbsp;</div><div>b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+["<div>a&nbsp;[] b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a <br></div><div>&nbsp;b</div>",
+     "<div>a&nbsp;</div><div>&nbsp;b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+["<div>a []&nbsp;b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a <br></div><div>&nbsp;b</div>",
+     "<div>a&nbsp;</div><div>&nbsp;b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+// These tests do not mind about the white-space sequence because it's not
+// important here.  That's tested by
+// editing/other/white-spaces-after-execCommand-*.tentative.html
+["<div>a&nbsp;[]&nbsp; b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a <br></div><div>&nbsp; b</div>",
+     "<div>a&nbsp;</div><div>&nbsp; b</div>",
+     "<div>a <br></div><div>&nbsp;&nbsp;b</div>",
+     "<div>a&nbsp;</div><div>&nbsp;&nbsp;b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+["<div>a&nbsp;[] &nbsp;b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a <br></div><div>&nbsp; b</div>",
+     "<div>a&nbsp;</div><div>&nbsp; b</div>",
+     "<div>a <br></div><div>&nbsp;&nbsp;b</div>",
+     "<div>a&nbsp;</div><div>&nbsp;&nbsp;b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+["<div>a []&nbsp; b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a <br></div><div>&nbsp; b</div>",
+     "<div>a&nbsp;</div><div>&nbsp; b</div>",
+     "<div>a <br></div><div>&nbsp;&nbsp;b</div>",
+     "<div>a&nbsp;</div><div>&nbsp;&nbsp;b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+["<div>a&nbsp;&nbsp;[]&nbsp; b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a&nbsp; <br></div><div>&nbsp; b</div>",
+     "<div>a&nbsp;&nbsp;</div><div>&nbsp; b</div>",
+     "<div>a &nbsp;</div><div>&nbsp; b</div>",
+     "<div>a&nbsp; <br></div><div>&nbsp;&nbsp;b</div>",
+     "<div>a&nbsp;&nbsp;</div><div>&nbsp;&nbsp;b</div>",
+     "<div>a &nbsp;</div><div>&nbsp;&nbsp;b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+["<div>a&nbsp; []&nbsp; b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a&nbsp; <br></div><div>&nbsp; b</div>",
+     "<div>a&nbsp;&nbsp;</div><div>&nbsp; b</div>",
+     "<div>a &nbsp;</div><div>&nbsp; b</div>",
+     "<div>a&nbsp; <br></div><div>&nbsp;&nbsp;b</div>",
+     "<div>a&nbsp;&nbsp;</div><div>&nbsp;&nbsp;b</div>",
+     "<div>a &nbsp;</div><div>&nbsp;&nbsp;b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+["<div>a &nbsp;[]&nbsp; b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a&nbsp; <br></div><div>&nbsp; b</div>",
+     "<div>a&nbsp;&nbsp;</div><div>&nbsp; b</div>",
+     "<div>a &nbsp;</div><div>&nbsp; b</div>",
+     "<div>a&nbsp; <br></div><div>&nbsp;&nbsp;b</div>",
+     "<div>a&nbsp;&nbsp;</div><div>&nbsp;&nbsp;b</div>",
+     "<div>a &nbsp;</div><div>&nbsp;&nbsp;b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+["<div>a&nbsp;&nbsp;[] &nbsp;b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a&nbsp; <br></div><div>&nbsp; b</div>",
+     "<div>a&nbsp;&nbsp;</div><div>&nbsp; b</div>",
+     "<div>a &nbsp;</div><div>&nbsp; b</div>",
+     "<div>a&nbsp; <br></div><div>&nbsp;&nbsp;b</div>",
+     "<div>a&nbsp;&nbsp;</div><div>&nbsp;&nbsp;b</div>",
+     "<div>a &nbsp;</div><div>&nbsp;&nbsp;b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
+["<div>a &nbsp;[] &nbsp;b</div>",
+    [["defaultparagraphseparator","div"],["insertparagraph",""]],
+    ["<div>a&nbsp; <br></div><div>&nbsp; b</div>",
+     "<div>a&nbsp;&nbsp;</div><div>&nbsp; b</div>",
+     "<div>a &nbsp;</div><div>&nbsp; b</div>",
+     "<div>a&nbsp; <br></div><div>&nbsp;&nbsp;b</div>",
+     "<div>a&nbsp;&nbsp;</div><div>&nbsp;&nbsp;b</div>",
+     "<div>a &nbsp;</div><div>&nbsp;&nbsp;b</div>"],
+    [true,true],
+    {"insertparagraph":[false,false,"",false,false,""]}],
 ]