The warning is displayed because you have both the "Resize components when app is resized" option checked and have written a SizeChangedFcn callback on a container. These two things are mutually exclusive, you must choose one or the other. If you have both, the SizeChangedFcn callback will be ignored.
AutoResizeChildren is a property that corresponds to the "Resize components when app is resized" checkbox in the "uifigure properties" panel (to see this panel, click on the edge of the uifigure, this panel is by default in the lower right side).
- Code-wise, when this checkbox is checked, the AutoResizeChildren is set to 'on' on the uifigure and on all the containers inside that app.
- Behavior-wise, when the checkbox is checked, once you run the app and then resize it, components inside your app (and inside all the containers) will be resized based on some heuristics.
Now, containers also have a SizeChangedFcn property that allows you to specify how components should be resized when the container itself is resized.
You should either choose to have the components resized based on the heuristics (i.e. check that checkbox), or write SizeChangedFcn callbacks on the containers, but not both. If you have written a SizeChangedFcn function on any container inside that app or on the uifigure, and checked the AutoResizeChildren option, the SizeChangedFcn function(s) will be ignored and components will be resized based on the heuristics. The warning tells you that the SizeChangedFcn will be ignored.
If you want the SizeChangedFcn callback(s) to work, you must uncheck the "Resize components when app is resized" checkbox.
Note: Even if you have removed the content of a SizeChangedFcn callback, the callback is still defined. If you want to use AutoResizeChildren behavior, you must remove the SizeChangedFcn callback altogether (right click, delete the callback).