Version 3.18 has only a single change:
TeXShop contains an obsolete sync method called Search Sync, and a modern replacement by Jerome Laurens called SyncTeX. In recent versions of TeXShop, the obsolete Search Sync from the Preview Window to the Source Window randomly hangs, making TeXShop unresponsive This was supposed to be fixed in version 3.17, but it wasn't. Unfortunately, when the modern SyncTeX cannot find a match, it calls the old Search Sync, so SyncTeX can indirectly hang as well.
It is silly to waste time on an obsolete method, so in TeXShop 3.18, Search Sync from the Preview Window to the Source Window is disabled and does nothing. Most users will notice no change. Users who misconfigured SyncTeX will lose synchronization.
Users should check that
1) in TeXShop Preferences under the Typesetting tab, the "Sync Method" is set to SyncTeX;
2) in TeXShop Preferences under the Engine tab, the two configuration lines for "pdfTeX" each contain the following flags
--file-line-error --synctex=1
3) in TeXShop Preferences on the same page, the two "TeX + dvips + distiller" lines contain the following instruction
--extratexopts "-file-line-error -synctex=1"
The easy way to do this is to push the four "Default" buttons beside these four entries.
Best Answer
At least as of the 3.06 version of TexShop (and perhaps earlier), TeXShop has included a Find Panel that uses OgreKit, which is a Cocoa framework. (It was developed by Isao Sonobe, according to the TeXShop help center.)
Basically, this means that this find panel ends up being much more powerful, in particular because it can utilize regular expressions. Additionally, this OgreKit version of the find panel allows one to set the scope of any given search. The two options for setting the scope are "Entire File" and "Selection".
In order to use the OgreKit version of the find panel, you need to open TeXShop, go to Preferences, and, under the Source tab in the Preferences, change the Find Panel radio option to "OgreKit Find Panel". After changing this, you will have to restart TeXShop, but you should then be able to set the scope of a search to the text that you have selected.
Update:
However, based on my testing this feature as of TeXShop version 3.18, this feature does not seem to work with
find. For example, if one selects a block of text and then uses only thefindfunction of the find panel, it will search the entire file. This is true for any function that does not include "all". That is, the only functions that really seem to work with this scope option arefind allandreplace all. I'm not sure why this is the case. Nonetheless, you should be able to at least use those two "all" functions with the scope restrictor as of version 3.18 of TeXShop. I've just reported this to Richard Koch; I'll update this answer accordingly.