I can reproduce that on Windows, but not on macOS. Possibly there are issues focusing reader after switching a tab.

Probably related:
1) Open a PDF file in a Reader tab
2) Click on the Magnifying Glass icon "Find in Document"
-> It opens the search box with the cursor placed inside the search box, so that I can directly type the text to search:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/0fnnbfeyh5c9nuyxizn3.png
3) Close the search box
4) Click on the My Library tab
5) Click on the PDF tab
6) Click on the Magnifying Glass icon "Find in Document"
-> It opens the search box, but this time the cursor is not placed inside the search box. Typing text does not do anything. I need to click manually inside the search box to start typing the text to search:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/txcyvd3prn81cogpkv5m.png
7) Press Escape
-> The cursor is placed into the search box.

Yes, all the issues are probably related to the same problem, and it only occurs on Windows (though I haven't tested it on Linux). We'll investigate.

I also see a similar problem with the magnifying glass in the note editor, from My Library or from a Reader tab. The cursor does not get into the search box when opening it by a click on the icon after switching item or tab. It only gets into the search box after clicking inside the note.

Thank you. It is all fixed for the search in the PDF Reader.

Still a small problem for the search inside a note:
1) Click on a note in the Items Tree
2) Click on the Magnifying Glass icon in the toolbar of the note
-> The focus stays in the Items Tree instead of going to the search box.
https://s3.amazonaws.com/zotero.org/images/forums/u265723/y54c90m3lyts7o12yovy.png

If I click inside the note before clicking on the Magnifying Glass icon, it works correctly:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/h72qyyyw9q9v5zt22koy.png

We'll fix it. Thanks.