This is not a new issue but providing a reproducible case has not been possible until now. Today I was updated to build 355 and though I don't think this is version specific, I found a reproducible case that destroys the on-screen design (see screen shots)
The steps below represent a simple example with its own specific effect on the visual display. Following the same steps but in a more complex visual results in a different visual problem but at least its your starter for 10.
1) Start a new project and drag a grid panel into the design area.
2) Select one of the columns (it doesn't matter which)
3) Find the renderTpl property
4) [In error] Enter some invalid text - like the number 1
At this point I see the grid is no longer rendered. This in itself seems like a problem but not the one which is the subject of this post.
Fix the problem by removing the invalid content in renderTpl using the 'clear' button. Now the display is really messed up because there are two grids rendered. Only one of them 'works'. I'm only able to return to a correct display by closing the designer application and starting it again.
I'm not holding this out as a severe problem but as a reproducible case of a problem that has occurred since I first started using the designer 3 or so months ago. In these other cases I've not been entering invalid code in renderTpl but probably inadvertently entering invalid content in other properties.
As you encounter these, please report them. We are aware of the renderTpl and have a ticket to fix it. Because we use the actual library to render the widgets on the canvas, we are at its mercy a little bit.
It's our responsibility to validate the invalid content that you put into various properties before passing it onto the library. There are still cases where we need to work on this, this is what you've encountered.
Sencha Architect Development Team