You can make set multi-lined text in a button from within the startup function startupFcn().
The first line below creates two lines of text within the button.
The second line moves the button down and doubles its vertical size. Set the button width and the position of the top of the button from the DesignView.
% Code that executes after component creation
function startupFcn(app, arg1)
app.Button.Text = ['Really long',newline,'button name']; % or sprintf('Really long\nbutton name')
app.Button.Position([2,4]) = app.Button.Position([2,4]) + [-1,1].*app.Button.Position(4);
% If you'd rather the button size to expand upward rather than downward,
% Replace the 2nd line above with
% app.Button.Position(4) = app.Button.Position(4)*2;
end
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