BOINC Manager GUI app (7.8.6) is still built as 32-bit application.
# actually this is the only 32-bit app running on my mac.
When will it become 64-bit?

The recent OS X/macOS have been 64-bit OS with 32-bit compatibility, but Apple already announced to drop support for 32-bit app for macOS near future.
Then, the next update for High Sierra ( 10.13.4 in beta ) starts giving warning for 32-bit apps.
Link to the related article at AppleInsider
http://appleinsider.com/articles/18/01/24/macos-high-sierra-10134-warns-users-about-32-bit-softwares-impending-demise

MacOS 10.13.5 still reports Boinc Manager 7.8.6 as 32 bit. MacOS 10.14 beta is expected to be released to developers at the June WWDC with 10.14 public beta releases a few weeks later that will not permit 32 bit applications to run. Full release of 10.14 is anticipated in October or November. Time is growing short to get a full 64 bit release of Boinc Manager tested and into the hands of users.

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Beta's in June can use the BOINC 7.9.3 test version, if 7.10 hasn't been released by then. By October/November we're already testing for BOINC 7.12

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