Some people noticed that the style in the WP-Admin is messed up when make a mouse over each item. The menu will disappear when mouse over the links. The problem is occurring in Chrome Browser and the problem occurs from Slimming Paint which is enabled by default in Chrome 45. Disabling slimming paint fixes the issue.

To disable this feature,
Go chrome://flags/#disable-slimming-paint
Enable the “Disable slimming paint” option.
Ensure that the “Enable slimming paint” option below it is not turned on.
Then Relaunch Chrome.

If the problem is not solved, try to add a function in your function.php file of your theme

function admin_menu_fix() {
    echo '';
add_action('admin_head', 'admin_menu_fix');

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