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fix(windows): pre-multiply alpha when setting menu item icons to fix black background #290

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fix(windows): pre-multiply alpha when setting menu item icons to fix black background #290

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45 changes: 43 additions & 2 deletions systray_windows.go
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Original file line number Diff line number Diff line change
Expand Up @@ -20,7 +20,7 @@ import (

var (
g32 = windows.NewLazySystemDLL("Gdi32.dll")
pCreateCompatibleBitmap = g32.NewProc("CreateCompatibleBitmap")
pCreateDIBSection = g32.NewProc("CreateDIBSection")
pCreateCompatibleDC = g32.NewProc("CreateCompatibleDC")
pDeleteDC = g32.NewProc("DeleteDC")
pSelectObject = g32.NewProc("SelectObject")
Expand Down Expand Up @@ -751,7 +751,36 @@ func (t *winTray) iconToBitmap(hIcon windows.Handle) (windows.Handle, error) {
defer pDeleteDC.Call(hMemDC)
cx, _, _ := pGetSystemMetrics.Call(SM_CXSMICON)
cy, _, _ := pGetSystemMetrics.Call(SM_CYSMICON)
hMemBmp, _, err := pCreateCompatibleBitmap.Call(hDC, cx, cy)

var bi struct {
Size uint32
Width int32
Height int32
Planes uint16
BitCount uint16
Compression uint32
SizeImage uint32
XPelsPerMeter int32
YPelsPerMeter int32
ClrUsed uint32
ClrImportant uint32
}
bi.Size = uint32(unsafe.Sizeof(bi))
bi.Width = int32(cx)
bi.Height = int32(-cy) // top-down
bi.Planes = 1
bi.BitCount = 32
bi.Compression = 0 // BI_RGB

var bits unsafe.Pointer
hMemBmp, _, err := pCreateDIBSection.Call(
hDC,
uintptr(unsafe.Pointer(&bi)),
0, // DIB_RGB_COLORS
uintptr(unsafe.Pointer(&bits)),
0,
0,
)
if hMemBmp == 0 {
return 0, err
}
Expand All @@ -761,6 +790,18 @@ func (t *winTray) iconToBitmap(hIcon windows.Handle) (windows.Handle, error) {
if res == 0 {
return 0, err
}
Comment on lines 784 to 792
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Copilot AI Mar 22, 2026

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If DrawIconEx fails, the function returns without cleaning up hMemBmp, which leaks a GDI bitmap handle on the error path. Consider adding a DeleteObject proc and deferring cleanup after CreateDIBSection succeeds, then canceling the defer when returning the bitmap successfully.

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if bits != nil {
pixels := (*[1 << 30]byte)(bits)[:cx*cy*4 : cx*cy*4]
// Pre-multiply alpha channel before creating HBITMAP
Comment on lines +794 to +796
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Copilot AI Mar 22, 2026

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cx/cy are uintptr (returned from Proc.Call), but they’re used directly in the slice expression [:cx*cy*4]. Slice bounds must be int, so this won’t compile. Convert cx/cy to int (after sanity/overflow checks) and use those int values when computing the pixel buffer length and when passing sizes around in Go expressions.

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Copilot AI Mar 22, 2026

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The comment says “Pre-multiply alpha channel before creating HBITMAP”, but the HBITMAP (hMemBmp) has already been created at this point. Rewording to indicate this is done before returning/using the bitmap would avoid confusion for future maintenance.

Suggested change
// Pre-multiply alpha channel before creating HBITMAP
// Pre-multiply alpha channel in-place before returning/using the HBITMAP

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for i := 0; i < len(pixels); i += 4 {
a := uint32(pixels[i+3])
pixels[i] = byte(uint32(pixels[i]) * a / 255) // R
pixels[i+1] = byte(uint32(pixels[i+1]) * a / 255) // G
pixels[i+2] = byte(uint32(pixels[i+2]) * a / 255) // B
}
}

return windows.Handle(hMemBmp), nil
}

Expand Down
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