Up and down keys skip several lines

[iandunn]

Describe the bug

Sometimes, when using the up and down keys to navigate through text within a block, and across blocks, some lines will be skipped over.

To Reproduce

1. Paste the following into the code editor, then switch to the visual editor:

<!-- wp:preformatted -->
<pre class="wp-block-preformatted">function mark_as_encoded( $post_ID, $post ) {<br/>  if ( false == $this->encoded || 'revision' == $post->post_type ) {<br/>   return;<br/>  }<br/><br/>  delete_post_meta( $post_ID, 'syntaxhighlighter_encoded' );<em><br/></em>  add_post_meta( $post_ID, '_syntaxhighlighter_encoded', true, true );<br/>}</pre>
<!-- /wp:preformatted -->

<!-- wp:paragraph -->
<p>lorum ipsum blah blah blah here's a long url http://wp-develop.test/wp-admin/edit.php?s=foo&amp;post_status=all&amp;post_type=post&amp;action=-1&amp;m=0&amp;cat=0&amp;paged=1&amp;action2=-1<br/></p>
<!-- /wp:paragraph -->

<!-- wp:preformatted -->
<pre class="wp-block-preformatted"><em>/**<br/></em><em> * @param array $assoc_args Output format arguments.<br/></em><em> * @param array $fields Fields to display of each item.<br/></em><em> * @param string $prefix Check if fields have a standard prefix.<br/></em><em> */<br/></em>public function __construct( &amp;$assoc_args, $fields = null, $prefix = false ) {<br/>$format_args = array(<br/>'format' => 'table',<br/>'fields' => $fields,<br/>'field' => null,<br/>);<br/><br/>foreach ( array( 'format', 'fields', 'field' ) as $key ) {<br/>if ( isset( $assoc_args[ $key ] ) ) {<br/>$format_args[ $key ] = $assoc_args[ $key ];<br/>unset( $assoc_args[ $key ] );<br/>}<br/>}<br/><br/>if ( ! is_array( $format_args['fields'] ) ) {<br/>$format_args['fields'] = explode( ',', $format_args['fields'] );<br/>}<br/><br/>$format_args['fields'] = array_map( 'trim', $format_args['fields'] );<br/><br/>$this->args = $format_args;<br/>$this->prefix = $prefix;<br/>}</pre>
<!-- /wp:preformatted -->

<!-- wp:paragraph -->
<p></p>
<!-- /wp:paragraph -->

2. Click in the title to move the cursor there
3. Press the down key and watch where the cursor moves to. Repeat until you reach the end of the document. You'll notice that several of the lines have been skipped.

Expected behavior

Hitting the up or down key once should only move 1 line, and no lines should be skipped.

Screen capture

https://cloudup.com/cWtcPA1LbI2

Desktop:

• OS X 10.13.5
• Firefox Developer Edition 62.0b6
• Gutenberg master @ e697f72

[iandunn]

This may be a duplicate of #6524, but I'll leave it open for now in case this contains some cases that wouldn't be solved by the same fix.