After thinking for some time I found a “solution”.
Actually it's not a solution, it's just a request for a more general solution for such problems:
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Now that I have to make an app with a cool design I VERY often would need a fully transparent button (TsButton, TsSpeedButton or whatever…) combined with a set of images (+ an optional caption).
When you hover the mouse over it or press it, it simply shows the corresponding images just like TsButton where we can use HotImageIndex, PressedImageIndex etc.
But in any case it does NOT show it's borders, it only shows the images.
This way we could easily create buttons that have odd shapes. You simply get partly transparent png images for the normal/hover/pressed-state from a designer and wire it to the transparent button.
(in reality the buttons are rectangular, but the user only sees the image).
This is something that is REALLY missing in the AC-pack