Man, I didn't think I could do all that but I did! I'm not very familiar with channels but I sure didn't want to add another path to the mix so went ahead on the channel.
I think there may be some misunderstanding. While my update now permits the script to be run while a channel is active, the channel is not actually used (and an alternate path must be selected). This is just a matter of how GIMP disables commands in the menus based on the type of the active drawable, and while the script doesn't need either a layer or a channel if an alternate path is used, I didn't want the user to have to switch dialog boxes just so the script's command wouldn't be grayed out in the menus.
Question: What does Ctrl + Shift + Path/Channel to selection do differently from just clicking on the to selection button in those?
As you know, clicking on the button (with no modifier keys) replaces the current selection (if any) with the shape of the highlighted Channel or Path. Holding SHIFT down while clicking will ADD the shape of the channel or path to the current selection. Holding CTRL while clicking SUBTRACTs the highlighted channel or path from the current selection.
And to your question, holding down both CTRL and SHIFT while clicking on the red button will INTERSECT the highlighted channel or path with the current selection. This means that if a region (even a single pixel) is selected in the channel (or path) AND also appears in the current selection then that region will appear in the resulting selection.
In the following image, the red square represents the selection before clicking on the button, the blue square represents the shape of the channel or path, and the green shape on the right shows the result. Note that these modifier keys provide the same functionality commonly available in the selection tools (as described in section 2.1 of the GIMP Users Manual
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