Reset The Zoom Level
If you have a problem with the flatplan locking, it's generally an issue specific to the computer you are using and can usually be resolved by resetting the zoom level to 100%.
On chrome this is found to the left of the address bar - click on the magnifying glass and click reset. Or use the Ctrl + and - keys to return the zoom level to 100%.
then reload the page by clicking the reload this page icon.
If resetting the zoom level does not solve it, then the issue is most likely to do with ad blockers on chrome or some sort of security set up on your computer.
View these notes for more details on addressing ad blockers
Try loading the page in a different web broswer - Safari, Chrome, Edge, or Brave.
Other solutions that have worked:
Change the display resolutions settings on your computer to 1280 x 720
Create a new user on the computer
If you have security software or anti-virus software installed - try removing that.
UPDATE JAN 2023
Google have released a version of Chrome that in some cases (depending on the resolution of your computer screen) will cause the flatplan to lock.
You need to reset the scale & layout to 100% to resolve this:
Right click on the Windows icon > Settings > System > Display > Scale & Layout set this to 100%