The zoom <100 issue is not reproducible anymore on Mac 10.14, Ubuntu 20.04 and Windows 10, hence marking as WFM. Additional investation and spin-off into another issue needs to happen related to this, since on Mac 11, the gridlines are gone (seems to be recent regression 89 works fine).
Bug 1467434 Comment 1 Edit History
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The zoom <100 issue is not reproducible anymore on Mac 10.14, Ubuntu 20.04 and Windows 10, hence marking as WFM. Additional investation and spin-off into another issue needs to happen related to this, since on Mac 11, the gridlines are gone (seems to be recent regression - 89 works fine).