Switching to source and back creates invalid html

[Satya Minnekanti]

To reproduce the defect:

1. Open any sample for CKEditor 4.0, clear the screen.

2. Type the paragraphs of text.

3. Switch to source mode and you will see the following.

<p dir="ltr">This is line 1.</p>

<p dir="ltr">This is line 2.</p>

4. While in source mode add a <br /> between the two p's as shown below.

   <p dir="ltr">This is line 1.</p>
   
   <br />
   
   <p dir="ltr">This is line 2.</p>
   

5. Switch out of source mode and then back to source mode and you will see the following invalid html.

<p dir="ltr">This is line 1.</p>

<p dir="ltr">&nbsp;</p>

<p dir="ltr">This is line 2.</p>

<p dir="ltr">&nbsp;</p>

Issue: Notice that <br /> tag been removed & original html is modified by the insertion of p blocks with &nbsp;'s.

This is very problematic since we have many docs that exist with embedded br's and every time we save & reload them, they become more and more corrupt.

This works in 3.6.6 but after step 4 go to Rich Text mode and keep your cursor in between the 2 paragraph's

See that empty space br get's removed

[Piotrek Koszuliński]

First bad commit ​git:f7f53df. It was ported from v3's commit [7526].

[Piotrek Koszuliński]

[7526] was reverted in v3 by [7676] (#9167). It should have been reverted in v4 as well.

[Frederico Caldeira Knabben]

The v3 issue was much broader. Not only the <br> was adding a <p>, but it was also keeping the <br> in place. It was really broken.

While working on #8167, I recall we undestood that the v4 behavior is the right one, but it was difficult to achieve it with v3. Therefore, we agreed to have a different behavior on v4, so [7526] must not be reverted here.

The current v4 behavior is desired. It transforms invalid orphan inline elements into block elements.