The Parsedown library has been patched to prevent tab indentations from being converted to spaces:

For example, if you had used the markdown syntax for displaying source code in your post, Parsedown had replaced your semantically correct tab indentations with hard spaces. This behavior meant that the code that was displayed after parsing was no longer exactly the code you inserted into the content editor. If someone had copied source code from your post into his IDE, then your tab indentations were gone because of the spaces.

That's why I hacked the Parsedown library and created a patch:
https://github.com/erusev/parsedown/issues/508

1 files changed, 10 insertions, 21 deletions

diff --git a/core/namespace/Parsedown.php b/core/namespace/Parsedown.php
index f5dd0fa..ec7bdde 100644
--- a/core/namespace/Parsedown.php
+++ b/core/namespace/Parsedown.php
@@ -131,23 +131,6 @@ class Parsedown
                 continue;
             }
 
-            if (strpos($line, "\t") !== false)
-            {
-                $parts = explode("\t", $line);
-
-                $line = $parts[0];
-
-                unset($parts[0]);
-
-                foreach ($parts as $part)
-                {
-                    $shortage = 4 - mb_strlen($line, 'utf-8') % 4;
-
-                    $line .= str_repeat(' ', $shortage);
-                    $line .= $part;
-                }
-            }
-
             $indent = 0;
 
             while (isset($line[$indent]) and $line[$indent] === ' ')
@@ -298,9 +281,12 @@ class Parsedown
             return;
         }
 
-        if ($Line['indent'] >= 4)
+        $conditionA = $Line['indent'] >= 4;
+        $conditionB = substr($Line['body'], 0, 1) === "\t";
+
+        if (($conditionA and $remove = 4) or ($conditionB and $remove = 1))
         {
-            $text = substr($Line['body'], 4);
+            $text = substr($Line['body'], $remove);
 
             $Block = array(
                 'element' => array(
@@ -319,7 +305,10 @@ class Parsedown
 
     protected function blockCodeContinue($Line, $Block)
     {
-        if ($Line['indent'] >= 4)
+        $conditionA = $Line['indent'] >= 4;
+        $conditionB = substr($Line['body'], 0, 1) === "\t";
+
+        if (($conditionA and $remove = 4) or ($conditionB and $remove = 1))
         {
             if (isset($Block['interrupted']))
             {
@@ -330,7 +319,7 @@ class Parsedown
 
             $Block['element']['text']['text'] .= "\n";
 
-            $text = substr($Line['body'], 4);
+            $text = substr($Line['body'], $remove);
 
             $Block['element']['text']['text'] .= $text;