Cool VL Viewer forum

I cannot reproduce any such issue here: when using SHIFT LEFT or SHIT RIGHT, my avatar does move in a perpendicular axis when compared to its original heading (easily seen with the mini-map open), whatever the cardinal point for it...

Maybe you got some AO or scripted attachment that is causing this ?... Or perhaps a window manager shortcut key or driver interfering with SHIFT+LEFT/RIGHT keys ? Or perhaps a camera scripted override that makes it looks like your avatar is not moving in the right direction ?

As another data point, I closely observed 'side walking'. Facing perfectly at any of the four cardinal directions (NSE) and shift arrowing (left or right), I do not see any deviation. In fact, the region boundaries I have turned on in the mini-map never veer from a perfectly straight line, no matter which of those four directions I'm facing. In addition to the suggestions from Henri, you might check your keyboard. It could possibly be sending both up (or down) in combination with your left or right presses. I can repro what I think your behavior is by holding say shift+left+up and I will walk 'diagonally'.

I eliminated every possibility of this being due to anything scripted, but I never tried if changing window managers made any difference.
In Firestorm I use the setting "Remap shortcuts already used by Linux", but who knows. I better browse that panel with Linux shortcuts again.)

(I perceived this in Firestorm, The Cool VL Viewer, Singularity, Kokua and the Lindenviewer under Wine so far.)