I don't get this with either macOSx or qt5Agg on my mac. I don't understand what could cause this, or why the order would make a difference (the subtitle gets placed the same regardless).

@anntzer are you using a tiling window manager? I have seen issues with tiling managers some getting out of sync with figure sizes.

That said, with qtile I can not reproduce this.

No, I am using a boring defaultish KDE.
I confirm I observe this with the following combos:

• master + {qt5,gtk3,wx,tk}agg (fedora+KDE)
• 3.2.0 + qt5agg (idem)
• master + qt5agg (windows)

I don't get this with #17494 either. It changes how colorbars are handled, so maybe worth checking out

#17494 appears to fix the problem; I guess that's a good argument in favor of it :)
Also, looks like (on master) the behavior is slightly different depending on whether full-screen is toggled by pressing F or by double clicking the title bar (but both are wrong).

Curious. OTOH I can't get this at all, using F, or maximizing w/ the green dot, so I can't really help. I would guess its something wrong w/ how your window manager reports back sizes when at full screen, but not sure?

I can reproduce this, and #17494 does fix both maximizing and fullscreen.

Actually, this is independent of fullscreening and just depends on the actual aspect ratio of the window (and thus indirectly on the aspect ratio of a full-screen window, with or without the titlebar as that is removed by "F"): I can repro the same issue with just

from pylab import *

fig = plt.figure(figsize=(8, 4), constrained_layout=True)  # <- note the change in size
fig.suptitle("foo")
axs = fig.subplots(1, 2, sharex=True, sharey=True)
im = axs[0].imshow([[1, 2], [3, 4]])
fig.colorbar(im, ax=axs[0], orientation="horizontal")
im = axs[1].imshow([[1, 2], [3, 4]])
fig.colorbar(im, ax=axs[1], orientation="horizontal")
plt.show()

and no window size manipulation. (The fact that the result depends on whether suptitle() is called first or last remains, though.)

With #17494 I now get a different issue wrt colorbars:

OK, fair enough - I can reproduce that. I guess we just have very different aspect ratio screens.

Not sure we should fix this since there is an easy work around, and I'd prefer to get #17494 to work.

WRT the overlapping colorbars, I can work on that in the context of #17494.... Colorbars are the hardest thing about the layout managers.