Can I leave Thailand while my visa extension application is under consideration?

And you must return before the date you've been given to get the extension stamped in your passport.. if you don't your re-entry permit will expire and upon re-entering the country you will be get a 60-day exempt stamp.. effectively cancelling your non-o visa.. requiring you to begin the process all over again.. starting with an application for a new 90-day non-o visa.. it's only a 4-week period and unless it's essential travel it's really ill-advised.. if you get stuck overseas unexpectedly.. eg. through a medical emergency or an aviation anamoly affecting your return flight.. you will have 4 new visa applications to handle.. with everything that entails.. and anyone currently using the income method would have to switch back to the capital method for the financials.. since it won't be processed as an extension.. so you need to factor that risk in to your decision to travel.. however unlikely you may consider it.. unless you are visiting a neighbouring border country i guess.. which would facilitate re-entry on foot if it became necessary.. but for me personally, any risk of prejudicing my visa status is too much risk!

As a rule there's no issue leaving during an under consideration stamp although when you apply for the extension let them know you'll be leaving in case they decide to do a home visit so you can coordinate it with them

AND

As James said, make sure you get a re-entry permit (which will be valid only for that u/c stamp) 🙂

ASK your local Thai Immigration office. It's up to them. Make sure you have a reentry permit if you do leave Thailand.