GUI: dialog box delayed closing

I'm writing a GUI in which I need a dialog box to show up under certain conditions. The problem is that closing the dialog box briefly freezes the GUI, for anything from a second to 5-6 seconds. The smallest example I've come up with is the following:

    function testdlg(varargin)

    if nargin >= 1 && isnumeric(varargin{1})
        n = varargin{1};
    else
        n = 9;
    end

    main = figure;
    for i = 1:n
        for j = 1:n
            uicontrol('style', 'pushbutton', 'string', [int2str(i) ', ' int2str(j)], ...
                'units', 'normalized', 'position', [0.101*(i-1) + 0.0 0.06*(j-1) + 0.4 0.1 0.05], ...
                'callback', @displayDialog);
        end
    end

    if ~any(strcmpi(varargin, 'noax'))
        axes('units', 'normalized', 'position', [0 0.01 0.3 0.3]);
    end

    uiwait(main);

    end

    function displayDialog(varargin)

    d = dialog('name', 'Dialog');
    uiwait(d);

    end

To describe this briefly: the function creates a grid (9x9 by default) of push buttons and a set of axes in a figure. A callback function is used so that clicking on any of the push buttons opens an empty dialog box.

What happens is the following. If the callback is never called (i.e., no button is clicked) and the figure is closed, everything is fast and there is no delay. If, however, a button is clicked and the dialog shows up, trying to close the dialog exhibits a long delay (about one second on my machine) before it actually closes. When attempting to close the figure after this, there is a similarly long delay for the figure to close. Any other GUI interaction is also delayed (so pushing a button to open another dialog box exhibits a one second delay as well).

Both the axes and the push buttons are important for this behavior. If you remove the axes (by calling testdlg noax), there is no delay. Reducing the number of buttons to a 5x5 grid or smaller (by calling, e.g., testdlg(5)) also gets rid of the delay. Since there is no noticeable delay in these cases, the delay I'm seeing can't just be due to the sum of the times it takes Matlab to handle one axes and the time it takes to handle >25 buttons: the delay I see is much longer than the sum of the delays I see when only the axes or only the buttons are displayed. So the effect must somehow be due to an interaction between the axes and the buttons.

Here's what I know from the tests I've done:

• this is not due to the uiwait calls: removing them completely does not affect the delay
• this is not due to the dialog box being modal (adding 'WindowStyle', 'normal' doesn't change anything)
• it's also not due to it being a dialog box (using a second figure instead of a dialog exhibits the same behavior)
• I've also tried setting all the axes options that are set to 'auto' by default to 'manual', with no success
• this is also not due to giving callbacks to all the push buttons; removing the callbacks and using an extra button for opening the dialog exhibits the same delays
• changing 'pushbutton' to 'popup' doesn't change the delay, though there is some change in the appearance of the dropdown menus after the dialog box is closed, which slowly reverts back to normal over a second or so
• using 'text' controls instead of 'pushbutton' or 'popup' does get rid of the delay.

Has anyone else noticed anything like this before? Is Matlab trying to do some layout calculations whose complexity grows quickly when axes are surrounded by many buttons? Is there a way to stop it from doing what it's trying to do?

The profiler shows a lot of time spent in Matlab's default closereq function that's called when the user attempts to close a window. All this does is essentially delete the figure. However, if I set my own CloseRequestFcn and try to time the delete call, it takes very little time.

I'm pretty stumped at this point -- if this is a bug, it's a very confusing one. Any ideas are very welcome. I'm running R2015b (8.6.0.267246) on Mac OS 10.11.6, if that's of any help.

[Geoff Hayes]

Tiberiu - I just tried your sample code (R2014a, Mac OS X 10.11.6) and did not observe any delay in closing either the dialog or main figure. Is there some other step that I may have missed?

[Tiberiu Tesileanu]

No extra step, for me just starting up a fresh Matlab instance and running this code exhibits the problem...

One thing that I did notice is that in fact the delay seems to start when the dialog is opened (see also the paragraph below), so that if I wait a couple of seconds before trying to close it, there is no extra delay for closing it. Similarly, after closing the dialog, if I wait a few seconds before trying to close the figure, there is no delay. So it seems like, on my machine, whenever there is a GUI interaction after and including the first opening of the dialog box, a countdown starts during which no other GUI operation is allowed.

Could you try to change the 'pushbutton' to a 'popup' and try this again? (You then have to click on a dropdown menu and select the only entry in order to open the dialog.) For me, when I open the dialog, there is a change in the appearance of the dropdown menus that takes a second or two to propagate over all of them -- by default, each of them has a pair of up and down arrows on a blue background at the right edge, but upon opening the dialog, the blue background disappears; when closing the dialog, the blue background reappears, again taking a second or two to do so for all the controls. Do you see this at all? It seems like the time it takes for this change in appearance is exactly the delay I notice.

Thanks for looking into this!

[Geoff Hayes]

Even if I change the pushbutton to a popup, I don't observe any performance issues.

[Tiberiu Tesileanu]

Thanks for trying, Geoff! I wrote to MathWorks support to see whether this is a general issue with R2015b, or whether it's just my computer. They're still looking into it.