Editor: Prevent wp.editor.autop() from doubling <br> tags inside captions - #14837

Description

Addresses Trac ticket #66373.

In wp.editor.autop() (src/js/_enqueues/wp/editor/base.js), <br /> tags followed by a newline (such as <br />\n or <br>\n) inside [caption] shortcodes were being converted into <br /><br />.

Root Cause

1. In wp.editor.autop(), <br> tags inside captions are temporarily converted to <wp-temp-br> to protect them from standard paragraph splitting:

   a = a.replace( /<br([^>]*)>/g, '<wp-temp-br$1>' );

2. Any newlines following those tags were retained.
3. Later in caption handling, all newlines were unconditionally converted to <wp-temp-br />:

   return a.replace( /\s*\n\s*/g, '<wp-temp-br />' );

4. As a result, <br />\n produced two adjacent <wp-temp-br /> tags (<wp-temp-br /><wp-temp-br />), which restored to <br /><br />. Each switch between the Visual and Text editor tabs doubled the number of <br> tags inside image captions.

Solution

Drop whitespace and newlines immediately following <wp-temp-br> tags before converting remaining standalone newlines into temporary breaks:

a = a.replace( /(<wp-temp-br[^>]*>)\s*\n\s*/g, '$1' );

Testing

• Added unit tests in tests/qunit/wp-admin/js/editor.js covering:
  • <br />\n and <br>\n inside captions.
  • Windows newlines (\r\n).
  • Attributes on <br> tags inside captions.
  • Standalone newlines without <br>.
  • Multiple lines with breaks inside captions.
  • Stability across repeated wp.editor.autop() and wp.editor.removep() editor toggle cycles.
• Enqueued editor.js and tests in tests/qunit/index.html.
• Verified all 712 QUnit tests pass in both index.html and compiled.html.
• Verified ESLint passes with 0 errors.

Props: @therssoftware, @yumiko.m
Fixes #66373.