When a layout is inserted into a page the LHS preview pane gains a horizontal scrollbar.
When there is no layout in the page there is no scrollbar.
Hovering over the blue grabber for the layout causes the scrollable width to reduce.
Steps to reproduce
On a clean Drupal 10.2.7 site using "standard" profile
Enable mercury_editor module
Set Mercury Editor Settings to use Basic Page bundle
Create a new Paragraph Type that uses Layout Paragraphs and One Column layout
Edit content type basic page; remove body field; add a field that references Paragraphs of the type just created.
Manage form display and set the new field to use layout paragraphs
Create new Basic Page
Use the RHS sidebar grabber to change the width of the main editing area to various widths and observe that no horizontal scrollbar is shown at any width.
Use the Mercury editor "+" button to insert a One column layout into the page.
Observe that:
Before adding the One column there was no horizontal scrollbar in the main editing panel.
After inserting the One column there is a horizontal scrollbar.
When hovering over the blue grabber for editing the layout the scrollbar disappears.
See attached video for a better description of the problem.