This is likely an upstream electron problem or something that apple changed recently.
If you have an old installer version you can try to download and install a fresh copy of obsidian from the website. Maybe it will help.
It started with 11.0 as far as I remember. Both my macOS and Obsidian installations are fresh, as in I didn’t upgrade them from an older system. I just tried to reinstall Obsidian anyway and I still have the same issue.
@WhiteNoise: I would recommend making the title bar light to conform with the rest of the light theme. From the screenshot below, you can see the Mac UI color changes quickly from light to dark to light again in three adjacent layers. It’s distracting. I searched for the default css in an effort to change it, but could not find any.
The following css snippet will help with the titlebar color. Instructions can be found at Customizing CSS. Basically, the .css snippet should be placed in .obsidian/snippets, which is hidden under the root folder of your vault.