So what's missing is both the numbers in the timeline (ruler) and the gridlines in the track editing area? Hmm, I haven't heard of that either, and I'm unable to duplicate it here (also iMac/Mojave)..
In case it's a display bug in GB itself, maybe try toggling any options related to the timeline/grid, like the Catch button (just to the left of the timeline), and the Snap to Grid and Alignment Guides options in the Edit menu; You could also try changing the grid format (Beats, Time) via the popup menu at the right in the Tempo display area (top center); And I guess you could also try resizing the window, on the off-chance it might redraw correctly. (May or may not help, but it can't hurt to try)
Other than that, there are the usual troubleshooting steps:
- Shutdown/restart the computer
- Blow out the Preferences
- Reinstall GarageBand
- Try with a different User (create a new user in System Preferences > Users & Groups to test,
and delete it afterward)
If your iMac is an older model, maybe Mojave/GarageBand doesn't play well with the graphics card?
I hope you're able to get it goingâ€”please do let us know if you find the solution.
Btw, I've moved this thread to the GarageBand subforum.