qtoolbox | qtoolbox is a set of tools for everyday Qt development

QToolBox uses a QVBoxLayout to show its contents, and adding a new item actually creates two widgets and adds them to that layout:

• a custom QAbstractButton used to display the title of each page and activate it;
• a QScrollArea that contains the actual widget;

One possibility is to create an animation that updates the maximum sizes of the previous and new items, but there's a problem: the button is connected to an internal function that eventually calls setCurrentIndex(), but since that's not a virtual function, it cannot be overridden, so it would be theoretically impossible to get mouse clicks and intercept.

Luckily, QToolBox provides a useful function that can help us: itemInserted() is called whenever a new item is added. By overriding that, we can not only get a reference to all the new widgets, but also disconnect the clicked signal of the button, and connect it to our own implementation of setCurrentIndex.

To simplify the animation process, I created a helper class that keeps references to the button, the scroll area and the actual widget, and provides functions that will control the height of the scroll area and properly configure it based on their current/future state.

Now, the animation works using a QVariantAnimation that interpolates from 0.0 to 1.0, and then uses that ratio to compute the proportion between the two widgets based on the target height. That height is computed starting from the current height of the tool box, then we subtract the size hint of the first button (and the layout spacings) multiplied by the number of pages. In that way we know, at any time, what would be the target height of the scroll area, even while resizing the window during the animation.
The function that updates that height is always called whenever items are added and removed, but also when the tool box is resized.

Special care is required for computing the sizes when the animation begins: by default, the layout bases its minimum size considering the minimum size hints of its widgets, and scroll areas always have a minimum height hint of 72 pixels. By default QToolBox immediately switches between pages by hiding the previous item and showing the new one, so the size hint is unchanged, but since we're going to show two scroll areas at the same time during the animation, that would force the tool box to increase its minimum size. The solution is to force the minimum size to 1 for both scroll areas, so that the minimumSizeHint will be ignored.
Also, since showing a widget adds a spacing, we need to decrease the size of the hiding scroll area by that amount until the new scroll area is taller than the spacing.

The only problem with all this is that to prevent too complex computations, clicking on another page during the animation has to be ignored, but if the animation is short enough, that shouldn't represent a big problem.

Probably the best way to do this is to use QDir.addSearchPath, which would allow you to use a simple alias in your stylesheet like this:

There are various issues with the provided UI.

The problem with the buttons has various reasons, and cannot be easily solved from Designer, especially because you changed a lot of properties (many of which are propagated, including stylesheets).

First of all, the left border is shifted because all layouts have a default margin in most systems, so you have to explicitly set it to 0.

In Designer, select each page from the object inspector, scroll to the bottom of the property editor and change the layoutLeftMargin property to 0. If the property is already 0 but is not shown in bold (which indicates whether a property uses the default value or an explicit one), change to another number and then set it again to 0. Then do the same with the frame you've added in that page.

The problem of the "incomplete" bottom border on the first frame is due to the fact that you used a QFormLayout, which in some styles makes button occupy only the minimum size, instead of making them expand.
Change it to a vertical layout, and then add the following to the side_menu stylesheet of the QPushButton selector:

No need to call the self.init_signals() within the derived class BPAApp.__init__() because it is already being called within the parent class BPAPOIApp.__init__(). When the parent class calls it, MRO would point it to the most derived implementation which is the derived BPAApp.init_signals() (which in turn calls the parent BPAPOIApp.init_signals()).

You have the following errors:

• QButtonGroup does not have the clicked signal but buttonClicked.
• You must not use ().

The syntax in general is: obj, & Class_of_obj, in your case backgroundButtonGroup is QButtonGroup and this is MainWindow so preliminarily the syntax is: