Update: Starting in Matlab r2020b, change the stack order of UI components in App Designer using the reorder tool (see release notes). ----------------------------------------------------------------------------------------
Unfortunately uistack() is not functional with UIFigures (prior to r2020b). Here's a function you could embed in your app that will put an object on top of the UI stack. Add the function by opening the app in app designer, go to code view, select functions, and add the function by pressing the green "+" under the Code Browser. Then call the function any time an object needs to be moved to top. Unfortunately the app image will flicker during the restack as the objects are redrawn.
function uistackTop(~, appfig, obj)
appHandles = appfig.Children;
hIdx = find(appHandles==obj);
newOrder = [hIdx,setdiff(1:numel(appHandles),hIdx)];
appfig.Children = appHandles(newOrder);
Example:
appfig = app.myAppFig;
obj = app.UIAxes2;
uistackTop(app, appfig, obj)
Also see this answer for a function that moves a UI object to the bottom of the stack. 
Best Answer