>>> When you do need to use the Web browsers that implement that UI, do 
>>> you frequently regret they don't work like Ivy?
>>
>> I don't understand your question.
>
> Which of the two do you like better: the Emacs modes like 
> icomplete-vertical or the drop-down list UI of the browsers?
>

The Emacs modes...  I guess I'm a hopeless case ;-)

>>> No, those browsers never drop a list which is as wide as the 
>>> window/frame.  So they waste much less screen space.
>>
>> I attach a screenshot of Chromium.  The frame is 2880 pixels wide, the 
>> "combo box" with completion candidates is 2472 pixels wide.  That's 86% 
>> of the frame width.  Okay, 86% is not 100%, but 86% is not "much less" 
>> than 100%.
>
> Sheer luck.  Try typing something into the Firefox's address bar
>

Sheer luck???  Most users type their queries directly in its address bar 
as I did, so they see exactly what is displayed on the screenshot, and the 
"market share" of Chromium/Chrome is 66%.

Here is a screenshot with Firefox, the combo box is 2342 pixels wide, that 
is, 82% of the frame width.

By the way, the search combo box in Chrome and Firefox is also taller than 
the minibuffer in Emacs: it uses 30% of the frame height in Chrome, and 
50% of the frame height in Firefox.  For Emacs (with the default 
max-mini-window-height value) it's 25%.  In terms of "wasted screen 
space":

- Emacs uses 1.15 M pixels
- Chrome uses 1.25 M pixels
- Firefox uses 2 M pixels

>
> or the small Google Search window to the right of the address bar.
>

This one I don't use, and AFAIK it is not "activated" by default, as it is 
redundant with the address bar.