dialog overlay hides page content when there is a message-strip

[pskelin]

Describe the bug
Opening a dialog causes the whole page content behind the overlay to disappear

To reproduce
Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/p9m5w08q80
2. Click the open button
3. The open button disappears

Expected behavior
the open button should be visible behind the overlay. If the message strip is removed from the page the button stays as expected, so this behaviour is influenced by the message strip shomehow.

Screenshots
If applicable, add screenshots to help explain your problem.

Context

• UI5 Web Components version: 0.10.1
• OS/Platform: MacOS
• Browser (if relevant): Chrome
• Other information: {...}

Affected components
ui5-dialog
ui5-messagestrip

[ilhan007]

That is because the message strip has height 100%;

the body grows when the dialog is opened, the message strip takes the whole height and the button disappears (although it`s just bellow the message strip), as the message strip takes the full view port).