Fix: Fixed PNG-based .ico files rendering with black backgrounds

[kalmix]

Resolved / Related Issues

• Closes Bug: PNG-based .ico files render with black background in sidebar and thumbnails #17923

What was wrong
Custom folder icons using PNG-based .ico files displayed with a black background instead of transparency in the sidebar and thumbnail previews. The issue occurred specifically when:

• Icons were rendered at small scales (sidebar icons) and Tab icon.
• Thumbnails were scaled down using Ctrl+Scrollwheel
• The .ico file was PNG-based.

Root Cause
The IsAlphaBitmap() method in Win32Helper.Storage.cs only detected semi-transparent pixels (0 < alpha < 255) but missed fully transparent pixels (alpha = 0).

// Only detected semi-transparent pixels
if (pixelColor.A > 0 & pixelColor.A < 255)
    return true;

The Fix
Changed the alpha detection condition to catch all non-opaque pixels:

if (pixelColor.A < 255)
    return true;

This ensures that any pixel with transparency (including alpha = 0) is detected, triggering the proper alpha bitmap conversion that preserves transparency.

Before and After Screenshots:

Before	After

Steps used to test these changes

1. Opened Files
2. Created a custom folder icon using a PNG-based .ico file with transparent background
3. Applied the icon to a folder via folder properties
4. Pinned the folder to the sidebar
5. Verified the icon displays with proper transparency
6. Scaled thumbnails down using Ctrl+Scrollwheel
7. Confirmed transparency is preserved at all sizes
8. Tested with BMP-based .ico files to ensure no regression
9. Tested with system .dll icons to confirm they still work correctly

[yaira2]

Does this have any impact for #14993?

[kalmix]

@yaira2 Yes, this change fixes that issue too. I've tested the change using the imageres.dll of a Windows 10 build.

Before

After

In the related issue (#14993) some icons do work with the old code, because they contain semi-transparent pixels (0<α<255) but for other icons such as the desktop icon that doesn't have any semi-transparent pixels the bug will appear.

[yaira2]

LGTM