Microsoft has released Windows 10 Insider Preview Build 18353 (19H1) to Insiders in the Fast ring. This release is mostly bug fixes, but does include improvements to the Windows Sandbox feature. Microsoft continues to add improvement to the feature. Hotkeys have been added to activate the ease of access dialog and for entering and leaving fullscreen mode. Microsoft also fixed an issue in memcpy that caused some drivers to hard-hang the system on load; this could manifest as a hang on upgrade.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-insider-preview-build-18353-released-with-sandbox-improvements/