Thank you for contacting us on this.
I'll be happy to help you!
You can follow the next steps to disable the Visualforce previewer for your TWS project:
please navigate to the menu Tools -> Options -> Projects -> Presentation; there you can find the 'Collapse Visualforce previewer' settings - please enable this setting and the previewer won't appear when you open any Visualforce page the next time.
Also, I want to draw your attention that the IDE remembers a state for the previewer in each Visualforce page file. So, for example, when you have some pages which are opened in the code editor with enabled previewer, and you enable the setting to collapse Visualforce previewer - TWS remembers this state for these pages and the previewer would appear the next time. However, if you collapse it - the IDE would remember this state also and the next time you shouldn't collapse it manually by sliding window to the top.
Hope this is what you are looking for.
Please let me know if you would have any additional questions.