Font Book crashes on macOS 26.0 due to corrupted showDetailPane key in user preferences

Thank you! This terminal command worked for me to open the app as well. Once the app was opened, I closed all detail pains and now I can open the app normally.

I did not have the "Thai Glyph" issue the OP posted, though. YMMV

Some EtreCheck users have been reporting similar problems. There's something in the window restoration logic of recent updates that leads to crashes. Most of my reports come from people using the most recent versions of Ventura.

Try holding down the shift key when launching the app. That should clear out the window restoration logic. It should launch normally after that, without holding down the shift key every time.

And if it happens again, try the shift key fix again.

I've 2 Macs. Only iMac 2020 experiences with this issue while MacBook Air M1 doesn't.

After update to Tahoe 26.0.1, I try running above command and it seems fix the issue. I can normally open Font Book multiple times without this problem again.

You can send a bug report from the feedback page

Feedback - macOS - Apple

I don't have the technical expertise to evaluate the crash report, but Font Book is crashing for me, too, in MacOS 26. Running the Terminal command worked. I submitted a bug report to Apple. I asked Claude.ai to explain the crash report, and this was the response:

What Crashed

• App: Font Book
• When: September 29, 2025 at 7:36 AM
• Your Mac: M4 Pro Mac mini running macOS 26.0

Why It Crashed

The app hit a programming error related to window layout and constraints. Specifically:

1. The Problem: Font Book's interface (built with SwiftUI) was trying to update how elements are positioned on screen
2. What Went Wrong: The system detected that views were improperly trying to update their layout constraints in a way that wasn't allowed
3. The Result: macOS deliberately crashed the app to prevent worse problems (this is called a "breakpoint exception")

The Technical Issue

Looking at the crash trace, the problem occurred when:

• SwiftUI was updating the toolbar preferences
• This triggered a cascade of view layout updates
• Something in the view hierarchy tried to update constraints at the wrong time
• The system caught this illegal operation and terminated the app

What This Means for You

• Your computer is fine - this is an app bug, not a system problem
• Your data should be safe - the crash happened during a UI update, not while working with fonts
• It's likely a bug in Font Book - particularly around how it handles toolbar and window layout updates

What You Can Do

1. Restart Font Book - it should work normally most of the time
2. Report it to Apple - use the crash reporter that likely appeared
3. Watch for patterns - if it crashes repeatedly doing the same thing, avoid that action until an update fixes it
4. Keep macOS updated - Apple may fix this in a future update

This type of crash is typically intermittent and shouldn't prevent you from using Font Book for most tasks.

I too have this exact issue, and the fix OP describes works just as well for a one off launch. I can't open Font Book via terminal commands every time I want to use it though.

Thank you! This worked for me!