QDockWidget can't be resized to zero width

When I resize a dock widget with a minimum size set as 0 x 0 and without any content, I can only resize it horizontally to some arbitrary value (79px) whereas 0px is expected.

When the dock widget is resized horizontally, there seems to be some size constraint that's not configurable in any way.
This happens whether the dock widget is docked to some area or floating.
The title bar text is reduced with an ellipsis, so this is not the limiting factor. The title bar buttons have also plenty of space.
What is this limiting factor and how can I make this dock widget resize to small widths?

Below is the dockwidget when the program starts:

When I try to resize it horizontally (by dragging the window border), it will only go up to a certain point horizontally.

The following is a simulation of the expected behaviour, a very small width. It cannot be drag-resized to this:

If I set the 'vertical titlebar' flag for this dock widget, the same thing happens but vertically. I believe that the title bar is related to this.

My intent is to use that dock widget to hold very small tool buttons, using the least screen space possible.
Is this a bug? How can I resize the dock widget to this size?