I'm not Ben, but here are my answer:
1. Is this the first time that you encountered this issue?
It happens repeadetly. I can reproduce this issue every single time. I open a document and this second broken window with only a formula bar appears when I minimize the window.
2. Did you make any changes on the computer before you encountered this issue?
No. I installed Office 365 and there it was. Right from the beginning. It wasn't there in 2010 version.
- What troubleshooting steps have you tried so far?
Open Google (or Bing) and search for 2nd Excel Window, Another Excel Window Opens etc. and whatever link you click if there is a solution, I've tried it, including repair, fresh install, changing some Excel settings, disabling add-ins, deleting add-ins,
running in safe mode etc., nothing fixes this issue. Please, keep in mind, that I can reproduce this in Safe Mode as well.
4. Have you tried disabling all the add-ins in the Excel app?
Sure I did. No luck. I can reproduce this bug in safe mode, too.
5. Does the issue only happen when using the Find and Replace feature?
It is not related to the use of any feature. This broken formula-bar-only window doesn't appear when you use a certain feature, it's already there, it just gets in the way when using a feature.
6. What is the current version of Office you are using?
It happened in every version of 2016 since February 2017 when I started using Office 365. My current version is:
Microsoft Excel 2016 MSO (16.0.8431.2046) 64-bit
Product ID: 00265-80115-81224-AA076
And fix this WYSIWYG editor here at Community pages. It's painfully slow and I'm using Edge!