"A fix committed last week removed an override of a private method in the Cocoa API, called _cornerMask. According to the author of the fix, the method calculates the shadow of the window, and overriding it "forces WindowServer to repeatedly recalculate and repaint the shadow." The likely reason the problem appeared only after the release of Tahoe is a new optimization, which this override then broke."
"This issue, combined with another relating to password AutoFill that can cause slow scrolling in Chrome and Chromium-based applications, means that some early Tahoe adopters have had a bad experience. Developers using the Rust-based Zed editor have also had Tahoe issues related to changes in AutoFill, the culprit being the same component that affected Chromium, a component called NSAutoFillHeuristicController. Apple has updated Tahoe to 26.01 in the last few days, which reportedly improves the AutoFill issue, though not completely for Chromium."
An override of a private Cocoa API method named _cornerMask in Electron forced the WindowServer to repeatedly recalculate and repaint window shadows, causing system-wide slowdowns on macOS 26 Tahoe. The problem likely emerged after Tahoe introduced an optimization that the override broke. Methods starting with an underscore are private in Apple's naming conventions. Performance degradation was most apparent when multiple Electron-based applications ran concurrently. A separate AutoFill issue involving NSAutoFillHeuristicController caused slow scrolling in Chromium-based apps and affected the Rust-based Zed editor. Apple released macOS 26.01 which improves AutoFill, while Electron backported fixes that require app rebuilds and redeployments.
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