https://codepen.io/sshenoy291988/pen/oNvXJxO?&editable=true
The messageBarButtom has a hard-coded width value because of which any longer button text does not render correctly . Eg - Changing the text from 'Yes' to'Reload' shows how the text overflows from the button boundary and breaks the text
Issue is probably here : https://github.com/OfficeDev/office-ui-fabric-react/blob/6ff7873e60436513ce7c3e4f39982a461fa5c86f/packages/office-ui-fabric-react/src/components/Button/MessageBarButton/MessageBarButton.styles.ts#L12
The messageBarButton text should be able to render any text without breaking the text into multiple lines
Are you willing to submit a PR to fix? (Yes, No) No
Requested priority: (Blocking, High, Normal, Low) Blocking
Products/sites affected: (if applicable) Yammer
I can check with the designers and see if there's a good reason to constrain this.
However the actual button implementation is passed in by each one of these examples and you have full access to what's actually displayed. If you want an immediate fix, you can render whatever button you'd like, or override the default styles like so:
<MessageBarButton styles={{ root: { width: 'auto' } }}>Reload And Don't Wrap Me</MessageBarButton>
Here's a CodePen with the styles overridden:

It appears the fixed width was added in #8779.
:tada:This issue was addressed in #10149, which has now been successfully released as [email protected].:tada:
Handy links:
Most helpful comment
I can check with the designers and see if there's a good reason to constrain this.
However the actual button implementation is passed in by each one of these examples and you have full access to what's actually displayed. If you want an immediate fix, you can render whatever button you'd like, or override the default styles like so:
Here's a CodePen with the styles overridden: