That’s right. If you have any extra spacer tile after the “real” tile, it will prompt and remove all the extra ones if you’d like to. Since there’s no way to know if a spacer is placed on purposed for the layout design or added accidentally if it is among the real tiles. The fixed canvas layout design is indeed in our list, so you feedback is greatly appreciated.
Meanwhile, you may want to try the following trick for editing tablet/mobile dashboard on a PC/laptop browser.
And @Stan_Silverman also shared his trick of using “Window Resizer”, a chrome extension, which allows you to change your browser resolution to your table with just one click, and makes editing tablet dashboard on a PC/laptop easier.