Font Book crashes on resize in the macOS 27 Golden Gate RC 26A428 (September 12, 2026)
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TL;DR
Font Book crashes when you resize its Sidebar or its Inspector on the macOS 27 Golden Gate release candidate, build 26A428, a Mac regression that dates back to developer beta 4 and was listed as still open on September 11, 2026 in a roundup by Michael Tsai. What is new on September 12, 2026 is a workaround: a release-candidate tester reported the app crashing on every launch after a single resize, and posted a defaults command that clears the stored pane state. Apple's release notes for the candidate contain no Font Book entry at all, so on that Saturday the people affected were beta and release-candidate testers on Apple silicon Macs.
- Kevin C, August 6, 2026: "As of #macOS27 beta 4, Font Book now consistently crashes when you try to resize either the Sidebar or the Inspector."
- The pane state is saved to preferences, so one resize can leave Font Book crashing on every launch.
- The workaround posted on September 12, 2026 is
defaults delete com.apple.FontBook showDetailPane. - It recovers the app and does not repair the bug: drag either divider again and the crash returns.
- Nothing about Font Book appears in Apple's release candidate notes, as a known issue or as a fix.
What broke
The original report is a Mastodon post from Kevin C on August 6, 2026, five weeks before this day, and it dates the regression precisely: "As of #macOS27 beta 4, Font Book now consistently crashes when you try to resize either the Sidebar or the Inspector." Consistently, not intermittently. The trigger is dragging a divider.
The same post suggests the ground was already uneven. Its author writes that he should perhaps be hopeful this means Apple had noticed an earlier, related problem in Font Book and made it worse while trying to fix it. That is the reporter's reading of the history rather than a documented regression path, and I would not put more weight on it than he does.
The post itself had no reblogs, no favorites and no replies, so its reach came entirely from being picked up later. Tsai's roundup of September 11, 2026 listed Font Book among the bugs still unfixed in the release candidate, five weeks after the first report.
How one resize makes the app unopenable
Font Book saves its pane layout to preferences, which means a crash caused by that layout can outlive the crash itself. A release-candidate user wrote in the MacRumors Forums thread on September 12, 2026: "Be careful resizing the Inspector or sidebar in Font Book. The app now immediately crashes for me anytime I try to open it."
That is the difference between an irritating bug and a lost application. The resize is a single action, the state is written to disk, and on the next launch Font Book reads that state and dies before you can change it back. Nothing in the interface offers a way out, because the interface never appears.
For anyone who installs, validates or organizes fonts as part of their work, that is the whole tool gone until the stored state is cleared by hand.
The workaround, and what it costs
The same forum post carries the recovery. In Terminal, run defaults delete com.apple.FontBook showDetailPane, then open Font Book again. That deletes the saved detail-pane preference, so the app starts from its default layout instead of the one that crashes it.
What it costs: your saved detail-pane and Inspector arrangement is gone, and more importantly the bug is untouched. Drag the Sidebar or the Inspector divider again and you are back where you started. This is a way to recover Font Book, not a way to use it normally, and there is no setting that makes the dividers safe.
I have seen no second method published. If the command does not bring the app back, nothing else in the reports covers that case.
What Apple has published
Nothing at all. Apple Developer's macOS 27 release notes, a page with no publication date of its own, contain no Known Issue and no Resolved Issue for Font Book anywhere. The AppKit resolved list runs to fifteen or more entries covering open and save panels, text fields, menus, segmented cells and gesture recognizers, and not one of them concerns font management.
So as of September 12, 2026 this shipped undocumented into the release candidate, unfixed and unacknowledged, the day after the developer roundup named it. Who that affects is narrow and worth stating plainly: people running macOS 27 Golden Gate beta 4 through release candidate 26A428, on Apple silicon Macs. If you are on macOS Tahoe, the Font Book being discussed is not yours.
One more item from the same forum page, unrelated and confirmed nowhere else I could check: "Text in Safari isn't draggable anymore. Possibly related to the little Apple Intelligence arrow that appears shortly after you select something." I am repeating that as a single user report, not as a diagnosis.
What to do today
1. On the release candidate, do not resize Font Book's Sidebar or Inspector. That is the entire trigger, and avoiding it is free. 2. If Font Book already crashes at launch, open Terminal and run defaults delete com.apple.FontBook showDetailPane, then reopen the app. 3. Expect the crash to come back if you drag either divider again, so leave the panes where they land.
4. If installing or validating fonts is part of your paid work, do it on a Mac that is not running the release candidate until this has a fix. There is no published fix to wait for yet, which makes a second machine the only reliable answer. 5. File it in Feedback Assistant with your build number. Apple's notes show no record of this bug, and a Known Issue entry would be the first evidence that it has been logged.
If you are on macOS Tahoe, there is nothing to do and nothing to install for this one. The step that matters for you is the one before the upgrade: know that Font Book on macOS 27 has an open crash with no fix, and decide the timing of your upgrade with that in hand.
Frequently asked questions
Will the defaults command delete my installed fonts?
No. It deletes one preference value, showDetailPane, which records how Font Book's detail pane was arranged. Your installed fonts and collections are stored separately and are not touched.
Is this fixed in the public release of macOS 27?
That is not knowable as of September 12, 2026. The bug is open in release candidate 26A428 and absent from Apple's notes, so there is no published fix and no statement about one.