Are you sure? to me it looks like there's room for one more, but maybe there's not. (Just realized there's a large border on the left, so it prob'ly wants as much on the right. <sigh> Guess you're right. Shame we can't handily get rid of excessive margins.)

But, since the thread has turned to "which buttons should be added / axed?", here's some input of what I think.

I think the zoom+ / zoom- buttons on the toolbar are not necessary, since that's already on the +/- hardware keys, so I wouldn't mind if they slid off onto the flyout, or even disappeared entirely.

And I think New File could be removed also, I never use it. Save is good to have handy, keep that one.
Wish there wasn't a GTK bug causing the double-wide, half-inactive color box.

Select rectangle should be made available too.

Text font would be nice, if we get text.

Last thing, though a bit harder, would be a flyout that gives us either the Pen Options, the Eraser Options, or the Highlighter Options, depending which is selected.
Or, maybe these could be long-press on the Pen Tool, Eraser Tool, and Highlighter Tool, and long-press on the Text Tool gives the Text Font, but that sounds even harder than an adaptive flyout.

So, I'm saying:
Axe

• New File
• Open File
• Zoom -
• Zoom +

Add

• Text Tool
• Text Font
• Select Rectangle
• Pen/Eraser/Highlighter Options (if feasible)
• Copy, Paste, Delete Flyout (for use with Select Rect)

And a quick question: what, if anything, does Enter (middle of dpad) do? It seems to do nothing. Possible useful behaviours:

• Toggle between zoom to fit page width and previous zoom level. (I draw zoomed in, and am often backing out, and then back in to draw more. This would also permit removal of Zoom Page Width button.)
• Toggle between most-recently selected tools. (Good for swapping pen/eraser.
• Go to finger-board to enter text. But how would it know the insertion point? Bad Idea!

Oh, one more gripe, though this one is for the creator of Xournal, nothing to do with your port: I wish the ruled paper were a certain spacing so that, say, a 20pt font would have the same pitch. It seems to be about 20.5pts?!

Thanks so much for the port!