[QGIS-Developer] What does the "Blur only alpha pixels" for label shadows mean?

[DelazJ]

Hi devs,

When setting a shadow item for a label there's a "Blur only alpha pixels"
option (see
https://docs.qgis.org/testing/en/docs/user_manual/style_library/label_settings.html#shadow-tab
)
I'm unable to understand what it's supposed to do and there's a report in
the docs source (https://github.com/qgis/QGIS-Documentation/issues/4264)
that mentions that it's a buggy feature. I can't tell but would like to
know if anyone understands what this should do, and whether it does it. ;(

Seems that interesting/starting point codes might be
https://github.com/qgis/QGIS/blob/129165a1001deb4b071c0b1d037e09d10f042ffc/src/core/qgstextrenderer.cpp#L1287
or
https://github.com/qgis/QGIS/blob/69e1b84b4c405a6b9fa7cfd2a72cf4c49a85f929/src/core/symbology/qgssymbollayerutils.cpp#L3722

Thanks for your help,
Harrissou
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