I might be wrong but I feel like macOS 26.1 slightly improved the situation. But still when transitioning back with my NSPageController to the collection view the pinned header is hidden until the transition completes but I was able to mitigate that issue by calling invalidateLayout() briefly after the navigation starts. I'll keep an eye on this one and also try our your approach - thanks for sharing!

The screenshots are from the sample code that I have also posted. The code is just to demonstrate the different behavior, which is unexpected. The first screenshot shows the menu when I add menu items via delegate -> NOK The second screenshot shows the menu when I add the items on toolbar item creation -> OK When I do the adjustments that you suggested, the problem would happen for both toolbar items, because then both menus would be updated in the menuNeedsUpdate method.

I'm a bit confused now. Isn't SVGs the recommended way to export my custom symbols that I created in the SF Symbols app? If not, what other way am I supposed to export my custom symbol? And if it's not supposed to work, why does the wiggle effect works as expected?

Are the parameters called "startDate" and "endDate"? They aren't documented and specifying these doesn't make a difference for me, the service keeps returning the forecast for the upcoming days. I am using the REST API. When I specify "dailyStart" (e.g. 2022-01-01T00:00:00Z) and "dailyEnd" (e.g. 2022-01-05T00:00:00Z), the service returns an empty "days" array.