If block mode is set to "Block input and screen", then, when a dialog box pops up on my dual monitor setup asking if I wish to postpone or skip, I see the box (correctly) appear on both screens. Unfortunately, only one of them actually works. Clicking on the buttons of the other one does nothing.

Actually, to be more precise, the buttons do nothing *until* I mouse over to the other monitor and then back again. 

Whether this happens or not depends upon whether the mouse is in the screen that is "below" the other display (in the xrandr sense) when the dialog box pops up. For example, I am able to switch which display exhibits the problem by using:

$ xrandr --output DVI-I-1 --above VGA-1
# Now I won't be able to click "postpone" or "skip" on my VGA-1 output if the mouse was already in that head when the dialog box appeared

$ xrandr --output DVI-I-1 --below VGA-1
# Now the bug affects only my DVI-I-1 output. 

The bug also occurs if the mouse is in a screen that is "right-of" another screen. 

Already ruled out: I have tested whether it matters if one of the displays is marked as "primary" and it has no effect. I also checked if it mattered which display had the notification area and that likewise had no effect. 

Since this bug only occurs when the screen is being blocked, it would be preferable if the dialog box only appeared on one monitor and the other screens were simply covered up.

*** Bug 1196 has been marked as a duplicate of this bug. ***