>> Emacs.Geometry: 100x59+2+2
>>
>> Without Cairo:
>>    Width: 818
>>    Height: 1022
>>
>> With Cairo:
>>    Width: 818
>>    Height: 962
>>
>> So for the same screen height, I gain 4 lines of text when using Cairo.
>
> What does ‘frame-geometry’ return for those frames?

Without Cairo:

((outer-position 2 . 2) (outer-size 820 . 1044) (external-border-size
1 . 21) (title-bar-size 0 . -20) (menu-bar-external) (menu-bar-size
818 . 17) (tool-bar-external) (tool-bar-position . top) (tool-bar-size
0 . 0) (internal-border-width . 1))

With Cairo:
((outer-position 2 . 2) (outer-size 820 . 984) (external-border-size 1
. 21) (title-bar-size 0 . -20) (menu-bar-external) (menu-bar-size 818
. 16) (tool-bar-external) (tool-bar-position . top) (tool-bar-size 0 .
0) (internal-border-width . 1))

And I attach two screen shots for with and without cairo.  To my eyes
it is the gap between lines that is different.

Yuan