Windows no longer get fully transparent background with winit v0.24.0 #1814
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msiglreith
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Feb 17, 2021
* Restore the ability to have fully transparent windows on Windows Besides its original purpose, commit 6343059 "Fix Windows transparency behavior to support fully-opaque regions (#1621)" also included some changes considered cleanups, one of them was: * Remove the `CreateRectRgn` call, since we want the entire window's region to have blur behind it, and `DwnEnableBlurBehindWindow` does that by default. But the original code actually disabled the blur effect for the whole window by creating an empty region for it, because that allows for the window to be truely fully transparent. With the blur effect in place, the areas meant to be transparent either blur the things behind it (until Windows 8) or are darkened (since Windows 8). This also means that on Windows 8 and newer, the resulting colors are darker than intended in translucent areas when the blur effect is enabled. This restores the behaviour from winit <0.24 and fixes #1814. Arguably, one might want to expose the ability to control the blur region, but that is outside the scope of this commit. * Remove useless WS_EX_LAYERED from transparent windows on Windows `WS_EX_LAYERED` is not supposed to be used in combination with `CS_OWNDC`. In winit, as it is currently used, `WS_EX_LAYERED` actually has no effect at all. The only relevant call is to `SetLayeredWindowAttributes`, which is required to make the window visible at all with `WS_EX_LAYERED` set, but is called with full opacity, i.e. there's no transparency involved at all. The actual transparency is already achieved by using `DwmEnableBlurBehindWindow`, so `WS_EX_LAYERED` and the call to `SetLayeredWindowAttributes` can both be removed.
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I have a small toy program that uses OpenGL (via glutin) in a non-decorated, transparent window to display some graphics. Starting with v0.24.0 the windows is no longer fully transparent. In the image below, the left side is with winit 0.23, and the right side with 0.24.
I identified #1621 as the culprit, reverting this commit fixes the issue for me, but I don't know Windows well enough to know why that is.
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