diff --git a/CHANGELOG b/CHANGELOG
index 7544c0162..350052bd0 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -9,6 +9,7 @@ CHANGELOG Roundcube Webmail
 - Templates: Make [space][slash] ending of condition objects optional (#6954)
 - Password: Make chpass-wrapper.py Python 3 compatible (#7135)
 - Elastic: Fix bug where it was possible to switch editor mode when 'htmleditor' was in 'dont_override' (#7143)
+- Fix so messages in threads with no root aren't displayed separately (#4999)
 
 RELEASE 1.4.2
 -------------
diff --git a/program/lib/Roundcube/rcube_result_thread.php b/program/lib/Roundcube/rcube_result_thread.php
index 2ebd48ca8..11adda667 100644
--- a/program/lib/Roundcube/rcube_result_thread.php
+++ b/program/lib/Roundcube/rcube_result_thread.php
@@ -573,9 +573,9 @@ class rcube_result_thread
             $end = strlen($str);
         }
 
-        // Let's try to store data in max. compacted stracture as a string,
+        // Let's try to store data in max. compacted structure as a string,
         // arrays handling is much more expensive
-        // For the following structure: THREAD (2)(3 6 (4 23)(44 7 96))
+        // For the following structure: THREAD (2)(3 6 (4 23)(44 7 96))((11)(12))
         // -- 2
         // -- 3
         //     \-- 6
@@ -585,8 +585,11 @@ class rcube_result_thread
         //         \-- 44
         //               \-- 7
         //                    \-- 96
+        // -- 11
+        //      \-- 12
         //
-        // The output will be: 2,3^1:6^2:4^3:23^2:44^3:7^4:96
+        // The output will be: 2 3~1:6~2:4~3:23~2:44~3:7~4:96 11~1:12
+        // Note: The "11" thread has no root, we use the first message as root
 
         if ($str[$begin] != '(') {
             // find next bracket
@@ -622,6 +625,7 @@ class rcube_result_thread
                         // @TODO: write error to the log or maybe set $this->raw_data = null;
                         return $node;
                     }
+
                     $p1 = strpos($str, '(', $off);
                     if ($p1 !== false && $p1 < $p) {
                         $off = $p1 + 1;
@@ -633,7 +637,26 @@ class rcube_result_thread
                     }
                 }
 
-                $thread = $this->parse_thread($str, $start + 1, $off - 1, $depth);
+                // Handle threads with missing parent by using first message as root
+                if (substr_compare($str, '((', $start, 2) === 0) {
+                    // Extract the current thread, e.g. "((1)(2))"
+                    $thread = substr($str, $start, $off - $start);
+                    // Length of the first token, e.g. "(1)"
+                    $len = strspn($thread, '(0123456789', 1) + 1;
+                    // Extract the token and modify it to look like a thread root
+                    $token = substr($thread, 1, $len);
+                    // Warning: The order is important
+                    $token = str_replace('(', '', $token);
+                    $token = str_replace(' ', ' (', $token);
+                    $token = str_replace(')', ' ', $token);
+                    $thread  = substr_replace($thread, $token, 1, $len);
+                    // Parse the thread
+                    $thread = $this->parse_thread($thread, 0, 0, $depth);
+                }
+                else {
+                    $thread = $this->parse_thread($str, $start + 1, $off - 1, $depth);
+                }
+
                 if ($thread) {
                     if (!$depth) {
                         if ($node) {
diff --git a/tests/Framework/ResultThread.php b/tests/Framework/ResultThread.php
index 5b6c7bafd..45c75ecc5 100644
--- a/tests/Framework/ResultThread.php
+++ b/tests/Framework/ResultThread.php
@@ -30,6 +30,7 @@ class Framework_ResultThread extends PHPUnit\Framework\TestCase
         $this->assertSame(false, $object->is_error(), "Object is error");
         $this->assertSame(1721, $object->max(), "Max message UID");
         $this->assertSame(1, $object->min(), "Min message UID");
+        $this->assertSame(731, $object->count(), "Threads count");
         $this->assertSame(1721, $object->count_messages(), "Messages count");
         $this->assertSame(1691, $object->exists(1720, true), "Message exists");
         $this->assertSame(true, $object->exists(1720), "Message exists (bool)");
@@ -37,6 +38,39 @@ class Framework_ResultThread extends PHPUnit\Framework\TestCase
         $this->assertSame(1719, $object->get_element('LAST'), "Get last element");
         $this->assertSame(14, (int) $object->get_element(2), "Get specified element");
 
+        $tree = $object->get_tree();
+        $expected = [
+            4 => [
+                18 => [
+                    39 => [
+                        100 => []
+                    ]
+                ]
+            ],
+            5 => [
+                6 => [],
+                8 => [
+                    11 => [],
+                    13 => [
+                        15 => []
+                    ],
+                    465 => []
+                ],
+                209 => []
+            ],
+            19 => [
+                314 => []
+            ]
+        ];
+
+        $this->assertSame([], $tree[1]);
+        $this->assertSame([], $tree[2]);
+        $this->assertSame([], $tree[14]);
+        $this->assertSame([], $tree[3]);
+        $this->assertSame($expected[4], $tree[4]);
+        $this->assertSame($expected[5], $tree[5]);
+        $this->assertSame($expected[19], $tree[19]);
+
         $clone = clone $object;
         $clone->filter(array(7));
         $clone = $clone->get_tree();