Screenshot tests: spectrum viewer size is non deterministic

[samtygier-stfc]

Summary

The size of the spectrum viewer seems to change non deterministically. This is causing failures of the screenshot tests, e.g. test_spectrum_viewer_add_new_roi. It can also be seen when running the application, e.g. with

python -m mantidimaging --path '/data/imaging/IMAT00010675_128/Tomo' -sv

or

In SpectrumViewerWindowView.__init__() the size always starts as:
self.size()=PyQt5.QtCore.QSize(905, 752)
some times it will stay at this size, or some times it will be resized in show()
self.size()=PyQt5.QtCore.QSize(943, 978)

Steps To Reproduce

python -m mantidimaging --path '/data/imaging/IMAT00010675_128/Tomo' -sv
or run the screenshot tests with
-k test_spectrum_viewer_add_new_roi

Expected Behaviour

Consistent size

Current Behaviour

Varies, but not clear why or when

Context

And as far as I can tell it is always tall at 5b827de (#2151), but non-deterministic at ea787c4

Commenting out QApplication.instance().setStyle(settings.value('theme_selection')) makes it always be tall. But size either side of that gives the short dimensions. So some complex interaction going on.

[MikeSullivan7]

It seems that putting QApplication.instance().setStyle(settings.value('theme_selection')) before compile_ui(ui_file, self) like:

class BaseMainWindowView(QMainWindow):

    def __init__(self, parent, ui_file=None):
        self._t0 = time.monotonic()
        super().__init__(parent)

        self._has_shown = False

        QApplication.instance().setStyle(settings.value('theme_selection'))

        if ui_file is not None:
            compile_ui(ui_file, self)

reliably creates the spectrum viewer at a size of self.size()=PyQt5.QtCore.QSize(914, 978)