When you press CTRL+s, I-search is activated, that provides a custom
toolbar with dedicated icons and lets one enter what to search in the
mode line.
Unfortunately, in the toolbar there is a help icon, shaped as a life
buoy, that is too big or at least inconsistently big wrt the other icons.
As a consequence, when I-search is activated the emacs windows
becomes bigger to accomodate space for the taller toolbar.
Not only having the window changing its size causes some inconvenience
because the text you are trying to focus on moves up and down. There is
a major problem if the emacs window is already vertically maximized
before you start searching. In fact, in this case there is no space for
the window to grow. As a consequence, the mode line goes out of the
screen and you cannot use it to search!
This behavior is system-dependent. For example,
I can't reproduce it on these configurations:
GNU Emacs 29.1.90 (build 1, x86_64-pc-linux-gnu,
cairo version 1.16.0)
GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit,
cairo version 1.16.0, Xaw3d scroll bars)
GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20,
cairo version 1.16.0)
Strange, the last is similar to your configuration:
In GNU Emacs 29.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.40,
cairo version 1.18.0)