bug symfony#16825 [VarDumper] fix .sf-dump z-index (debug bar conflict) (Antoine LA)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes symfony#16825).

Discussion
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[VarDumper] fix .sf-dump z-index (debug bar conflict)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#16824
| License       | MIT
| Doc PR        | -

Just changed the z-index because the twig dump is actually displaying over the debug bar (symfony#16824).

Commits
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a72efb9 [VarDumper] fixed .sf-dump z-index

Author: stof

src/Symfony/Component/VarDumper/Dumper/HtmlDumper.php

@@ -31,7 +31,7 @@ class HtmlDumper extends CliDumper
     protected $headerIsDumped = false;
     protected $lastDepth = -1;
     protected $styles = array(
-        'default' => 'background-color:#18171B; color:#FF8400; line-height:1.2em; font:12px Menlo, Monaco, Consolas, monospace; word-wrap: break-word; white-space: pre-wrap; position:relative; z-index:100000; word-break: normal',
+        'default' => 'background-color:#18171B; color:#FF8400; line-height:1.2em; font:12px Menlo, Monaco, Consolas, monospace; word-wrap: break-word; white-space: pre-wrap; position:relative; z-index:99999; word-break: normal',
         'num' => 'font-weight:bold; color:#1299DA',
         'const' => 'font-weight:bold',
         'str' => 'font-weight:bold; color:#56DB3A',