Changes proposed in this Pull Request:

• fix widgets extending outside the Customizer panel width fixes Customizer: Subscriptions widget has weird layout when selected #5811
• increase number of visible rows in text boxes for Subscription widget
• fix undefined indexes for title and url in WordPress Posts widget fixes Widgets: undefined indexes in Posts widget in Customizer #5893
• always load Milestone front end stylesheet so it's stylized when it's first added and previewed in Customizer, fixes Milestone Widget: add default styling in customizer #5907
• enqueue stylesheet for Customizer control for Milestone widget
• stylize controls for Milestone widget in admin screen and Customizer Fixes Milestone Widget: date field is not formatted properly #5889
  • Previously the styles were broken in mobile screen sizes, this patch fixes it.
  • It introduces distinctive separated labels for Date and Time in mobile screen sizes.
  • Balances height of elements and aligns them in admin and Customizer.
  • Increase number of visible rows in message text box.
  • Synced to wpcom in r147481-wpcom

View of the widget in Customizer.

View of the widget in Widgets admin screen in mobile screen size. The layout at this size in Customizer is similar.

Testing instructions:

• add these widgets:

Subscriptions
Authors
Gallery
Milestone
WordPress Posts

• make sure they don't extend beyond the Customizer width.
• the first time you add a Milestone widget to Customizer, make sure it's stylized in the preview
• the Milestone widget control in Customizer should be stylized
• add WordPress Posts in Customizer and make sure there are no PHP warnings regarding title and url array indexes

Proposed changelog entry for your changes:

Fix: adding a WordPress Posts widget in Customizer no longer displays PHP warnings
Fix: the Subscriptions and Authors widgets no longer extend beyond the Customizer width