Component is clipped when dragged outside of sheet in fixed-width view

Reproduction:

• open a channel in the channel editor
• switch to tablet or phone view
• drag a component and make sure part of it is outside of the sheet

Expected: the component should be completely visible, even when drag over the sheet's border

Actual: the component is clipped on the sheet's border

Note: the fix is easy (remove CSS rule overflow: hidden; on .channel-iframe-sheet selector) but that was added for CHANNELMGR-944 so we need to investigate if this (a growing horizontal scrollbar) no longer happens.