As it happens, the taskbar in Granular is tga & the resizing bar is bmp with magic pink with no display issues. Go figure
Just took a look, the gripper is bmp with magic pink but the resizing bar is just a bmp, no magic pink.
The gripper being bmp with magic pink I have never seen an issue with, and the resizing when magic pink is used seems to be more of an issue when taskbar is set to blur.
though I wonder if that's really a WFX issue, as I use WFX's 'dissolve' menu fade effect)
This may be; my desktop system has a lesser graphics card than your notebook and I always have issues when trying to use WFX menu and windows effects.