Currently I am working with both the Windows and the Linux version of Xournal (German) and I am quite happy that you did such a great job! Anyway I realized some strange behavior:
When reducing the window of Xournal I achieved fast, that the toolbar does not fit into the current window, which in general is no problem if you can reach the functions in an other way (as implemented in the drop down menu). But:
1) the lower toolbar does not care about missing functions until I reduced the window so that the icon for the white color disappears. Then the dropdown arrow is shown for the functions which cannot be displayed in the small window. I would have expect that the dropdown button appears as soon as the first function (selecting text style) disappears.
2) Furthermore the dropdown menu of the lower toolbar does not contain an entry for selecting the text style/size.
3) The next strange behavior appears in the upper toolbar when reducing the window to its minimum. In contrast to the lower toolbar, the upper toolbar knows its size and shows the dropdown button as soon as the first function (full screen) disappears. But this function obiously has no text representation because it is listed in the drop down menu as an empty entry.
4) I was wondering if I looked what was shown above this empty entry for the next function: "Suchen" (search function). This was obviously a wrong translation (caused by the magnifier icon). The function behind this text is adjusting the zoom.
5) I do not know if it is technically possible, but if I reduce the window to its minimum, all functions in the toolbar are listed in the dropdown menu without any seperator (as in the toolbar). It would be nice to have separators in the dropdown menu, too.
Greetings from an avid fan of your tool!