Visual editor hangs in Chrome and Firefox when a sequence of no-break spaces follows a paragraph

[greatislander]

Problem

A line of text wrapped in <p> tags followed immediately by ~30 no-break space characters (U+00A0) and additional text causes the visual editor to become unresponsive in Chrome and Firefox on macOS Sierra.

Steps to Reproduce

1. Insert the following content into the text editor of a post or page (note that the spaces preceding the asterisk are no-break spaces):

<p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.</p>
                               *

2. Save the post or page.

3. Switch to the visual editor.

Expected Behaviour

A WYSIWYG version of the content is displayed.

Actual Behaviour

The user receives the error message "The following page has become unresponsive. You can wait for it to become responsive or kill it." (Chrome) or "A script on this page may be busy, or it may have stopped responding. You can stop the script now, open the script in the debugger, or let the script continue." (Firefox). In some cases, letting the script continue will eventually load the visual editor properly.

Versions

• WordPress v4.6.1
• Twenty Sixteen v1.3
• PHP 7.0 + MariaDB
• macOS Sierra 10.12
• Google Chrome 53.0.2785.143 (64-bit)
• Firefox 45.0.2

[azaozz]

#38302 was marked as a duplicate.

[azaozz]

Still seems to be caused by "run away" backtracking in the regex that removes empty spaces from empty paragraphs, see #35890.

[azaozz]

In 39204:

TinyMCE: fix freezing the editor when wpautop is disabled and a paragraph starts with a lot of white space characters.

Fixes #38294.