>>> int main()
>>> {
>>>   xyzz = abcc;
>>> }
>>>
>>> 1. Place mouse over `main` and left double click - `main` is selected
>>> 2. isearch-forward (C-s) `xyzz` - `xyzz` is selected
>>> 3. place cursor over `x` of `xyzz` and mouse-yank-at-click (middle mouse
>>> down)
>>>
>>> You should see error "Lisp nesting exceeds `max-lisp-eval-depth`"
>>
>> It's an infinite recursion inside isearch-mouse-2.
>
> This is because let-binding overriding-terminal-local-map to nil
>
>   (let ((overriding-terminal-local-map nil))
>     (key-binding (this-command-keys-vector) t))
>
> doesn't disable its current value anymore.  It used to work when bug#23007
> was fixed in 2017.  So in Emacs 26.3 it correctly returned 'mouse-yank-primary',
> but in Emacs 27 it's broken and returns 'isearch-mouse-2' without ignoring
> the current value of overriding-terminal-local-map, i.e. currently let-binding
> 'overriding-terminal-local-map nil' has no effect for 'key-binding'.

In 2018 the logic of searching a key binding was changed.
Now it magically depends of the current mode instead of
more explicit keymaps as before:

Before 2018:

  (let ((isearch-mode t))
    (key-binding [mouse-2] t))
  => mouse-yank-primary

After 2018:

  (let ((isearch-mode t))
    (key-binding [mouse-2] t))
  => isearch-mouse-2

Anyway here is the fix: