Thanks for your try.
It might be an issue of either Lxde or your particular theme/theme engine.
(theme is Michaku, tried others without success,
)
Really strange because I've tried with gtk3, which works fine.
But I cannot select it for regular work due to many other annoying problems like changing size of components or broken ParentColor of TPanel.
Images
here, text in French but images are images, aren't ?
Well, big 1
st image is made with gtk3, left side is design mode, right side is running mode.
Next image shows gtk3 design, gtk3 running, gtk2 running, right panel is no more alClient.
Last image is gtk2 design mode with big size for font.