google_bookmarks_fix.user.js

// ==UserScript==
// @name         Google Bookmarks fix
// @match        https://www.google.com/bookmarks/*
// @grant        none
// @require https://code.jquery.com/jquery-1.12.4.min.js
// @require https://code.jquery.com/ui/1.12.1/jquery-ui.js
// ==/UserScript==

$(function() {
try {

    const MAX_SUGGESTIONS = 20;  // get first 20 results

    $("head").append (
        '<link '
        + 'href="https://code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css" '
        + 'rel="stylesheet" type="text/css">'
        + '<style>'
        + '  .ui-autocomplete {'
        + '    max-height: 600px;'
        + '    overflow-y: auto;'
        + '    overflow-x: hidden;'
        + '  }'
        + '  #ac-list { display: none }'
        + '</style>'
    );

    function split( val ) {
      return val.split( /,\s*/ );
    }

    function extractLast( term ) {
      return split( term ).pop();
    }

    /*
      Order of preference:
      1. words starting with term matching case
      2. words starting with term ignoring case
      3. words containing the term elsewhere matching case
      4. words containing the term elsewhere ignoring case
      5. other than that we rely on the order of the input list;
         in the case of Google Bookmarks it looks like it is
         alphabetically sorted

      Never return more than MAX_SUGGESTIONS results.
    */
    function getSuggestions( list, term ) {

      const lowerCaseTerm = term.toLowerCase();

      // prefer words starting with term
      let suggestions = list.filter( word => word.startsWith(term) );
      if (suggestions.length < MAX_SUGGESTIONS) {
        suggestions = suggestions.concat(list.filter(
            word => word.toLowerCase().startsWith(lowerCaseTerm) && !word.startsWith(term)
        ));
      }

      // words containing term anywhere inside
      if (suggestions.length < MAX_SUGGESTIONS) {
        suggestions = suggestions.concat(list.filter(
            word => word.includes(term) && !word.toLowerCase().startsWith(lowerCaseTerm)
        ));
      }
      if (suggestions.length < MAX_SUGGESTIONS) {
        suggestions = suggestions.concat(list.filter(
            word => word.toLowerCase().includes(lowerCaseTerm) && !word.includes(term) && !word.toLowerCase().startsWith(lowerCaseTerm)
        ));
      }

      return suggestions.slice(0, MAX_SUGGESTIONS);
    }

    $( "#bkmk_label_1, #gbqfq, .kd-textbox" )
      // don't navigate away from the field on tab when selecting an item
      .on( "keydown", function( event ) {
        if ( event.keyCode === $.ui.keyCode.TAB &&
            $( this ).autocomplete( "instance" ).menu.active ) {
          event.preventDefault();
        }
      })
      .autocomplete({
        minLength: 0,
        source: function( request, response ) {
          // delegate back to autocomplete, but extract the last term
          /*response( $.ui.autocomplete.filter(
            window.labelList, extractLast( request.term ) ) );*/
            const term = extractLast( request.term );
            const suggestions = getSuggestions( window.labelList, term );
            response( suggestions );
        },
        focus: function() {
          // prevent value inserted on focus
          return false;
        },
        select: function( event, ui ) {
          var terms = split( this.value );
          // remove the current input
          terms.pop();
          // add the selected item
          terms.push( ui.item.value );
          // add placeholder to get the comma-and-space at the end
          terms.push( "" );
          this.value = terms.join( ", " );
          return false;
        }
      });
}
    catch(e){console.log(e)}
});

Thanks for the great code and @planiko for easy instructions.

On line 10 I get the following
Parsing Error: Unexpected token $

I am guessing that might be because of the version of Chrome I am using?
Mine is: Version 67.0.3396.87 (Official Build) (32-bit)

Many thanks to Molhanec for writing the code and Planiko for the installation instructions.

Your Userscript works awesome! I have Tampermonkey and Violentmonkey in two different browsers and when I clicked on "Raw" on your userscript page in GitHubGist, it installed in both and it works great. Thank You.