• VSCode Version: 1.26.0 Insider (last commit date 2018-07-13)
• OS Version: Windows 10 April 2018 Update

Steps to Reproduce:

1. Ensure Underline keyboard shortcuts and access keys is checked in Windows' Make the keyboard easier to use options, under Ease of Access in the control panel.
2. In user settings, set the titleBarStyle to "custom" ("window.titleBarStyle": "custom").
   Note: with the titleBarStyle set to native, keyboard navigation doesn't work as expected either.

Does this issue occur when all extensions are disabled?:
Yes

Expected result:

• Access keys are visible at all times, i.e. even before pressing the Alt key.

After pressing the Alt key:

• First menu item is focused and access keys remain visible when Alt key is released.
• When pressing an access key, the corresponding menu option is triggered/opened.
• When opening a menu item with the arrow keys and then pressing an access key, triggers/opens menu item.

Actual result:

• Access keys are not visible before pressing the Alt key.

After pressing the Alt key:

• First menu item is focused, but access keys immediately become invisible when Alt key is released.
• Pressing access key doesn't do anything.
• When opening a menu item with the arrow keys, access keys are visible again, but pressing them does nothing.

Only when holding the Alt key the entire time, keyboard navigations works as expected.

Animated GIF's to compare behaviour with other Windows apps:
See the difference between Visual Studio Code and Visual Studio (the latter works as expected, although you can basically use any classic Windows application, for example Notepad works the same way too).

Visual Studio Code:

Note: I pressed down instead of the access key 'F' for File because the access key doesn't work.

Visual Studio: