Big Sur: Window is transparent on startup

[flit]

OS: macOS 10.11, reproduced on both x86_64 and AArch64

Prerequisites:

transparency enabled (< 1.0)
"keep window solid when MidiKeys is in front" enabled

Bug behaviour:

When MidiKeys first starts, the keyboard window can be transparent even though MidiKeys is the frontmost application.

MidiKeys initially sets the window to transparent in awakeFromNib:. Then it gets the applicationDidBecomeActive: notification and correctly sets the window to 1.0 alpha, confirmed by reading back the window's alpha. There is no subsequent deactivation notification. But for some reason the window remains transparent.