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99% regression from your fix of 39039. Please fix ASAP, it doesn't look good in dev build.
Ack. Will fix this afternoon...
*** Issue 39115 has been marked as a duplicate of this issue. ***
I think it's a simple problem that didn't cross my mind: The bitmap is built by calling GraphicsEnvironment....createCompatibleImage - which builds an image that is best compatible for fast copying onto the screen. It looks like on some platforms, the default image type has alpha, and some places it doesn't. My apologies...
Will disable buffering until I've solved the black background and tested it on multiple platforms.
Fixed in trunk. Bitmap buffer optimization turned off for now. Checking in core/windows/src/org/netbeans/core/windows/view/ui/tabcontrol/MetalUtils.java; /cvs/core/windows/src/org/netbeans/core/windows/view/ui/tabcontrol/MetalUtils.java,v <-- MetalUtils.java new revision: 1.5; previous revision: 1.4
verified in [nb_dev](200402241900)