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a standard feature of extensions is their ability to go 'full screen' when the user clicks on this icon:
is there any way to prevent this icon being shown/going fullscreen in a custom extension?
Again, there is nothing specificity built into sencha that allows me to just target one widget's zoom button using css selectors. Also there is not a parent css selector that exists realistically today, so you mean have a look at Child/Sibling css selectors.
THE PROBLEM is any class you make inside sencha is INSIDE what needs to be selected, namely the zoom button.
See the image below that illustrates this point. As you can see the class i add at the TOP level of the widget is still INSIDE the wrapper that contains the zoom button. Thus there is no way to uniquely identify that zoom button.