Trying to find text on a page with only un-selectable text (-webkit-user-select: none) fails to find any of the matches. Finding text on a page with mixed sections of un-selectable text and normally selectable text behaves more oddly. The first match in the section of un-selectable text is marked, but won't be highlighted when cycling through the matches: there will be one "pause" with no match highlighted. Any other matches in that section won't be found at all. To see the first problem, load the attached body-user-select.html, cmd-F, and look for the letter t. notice "not found". To see the second problem, load the attached user-select.html, cmd-F, and look for the letter t. Notice that only one t in the middle line is marked. Then cmd-G repeatedly to cycle through the matches and notice the "pause" when the t on that middle line would have been the active match.
Created attachment 21231 [details] Entire body is -webkit-user-select: none
Created attachment 21232 [details] Second line is -webkit-user-select: none
<rdar://problem/8081660>
I'm running into this bug as well. I'm on OS X 10.6.6 using WebKit nightly r76640.
To resolve this on iOS, we simply need to fix 251709. To resolve this on macOS, additional work is required, since macOS only uses the current selection for find (doesn't store ranges as done on iOS).