I'm having some graphical glitches with Preview.app and Quick Look after installing the beta. Everything else is working fine.
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Addendum** ---> possibly
faster fix for missing dictionaries : Full Credit to "mallen6," Apple developer forums --> "Hold shift key while opening the dictionary.app in application folder"
If the above doesn't work, try my procedure as described below; I found it to work without a glitch.
I found a fix for "Quick Lookup" not working in 10.13.4 beta; it may work in any version of 10.13.X, or earlier, if the "Lookup" function is problematic, or additional dictionaries are needed.
I used EasyFind.app to search for files with word "quick" and/or "Lookup" in their title.
List of files names with word "quick and/or lookup" (as in quicklook) was listed in QuickFind's search result window.
Highlight all "quicklook and/or lookup" files ending with ".plist" suffix, and delete them. Warning: only delete "quicklook and/or lookup" files which end in ".plist" suffix, which signifies "preferences" file. No restart required.
I then highlighted a word in TextEdit document, (but it works in any document or a website), and control-clicked the highlighted word. I chose "Lookup" from resulting dropdown menu. In few moments a "preference" window opened, listing available dictionaries, and begun a download. Once download of dictionaries completed I had a fully functional Lookup.
Adjust priority for dictionaries to be used in the dictionary listings; if left unchanged, your first results would be from wikipedia, followed by other choices.
I did not find a fix yet, for Preview scrambling pictures on launch.
Hope this is of help.
Macbook Pro 5,2 17 inch. (mid 2009)