GPU preview gets disabled
In Illustrator version 26.0.2 the GPU preview is disabled when rulers are shown or you enter outline or isolation mode. This bug has been around since at least v24.
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I can’t reproduce this bug too, so it can be a machine-specific case.
I cannot reproduce this bug.
GPU preview works for me whether I have rulers visible or not, in all view modes - normal view, outline view and isolation mode. I can switch it on and off at will with Ctrl+e.
I even get GPU driver warning when starting Illustrator.