And I was experiencing that with Joplin 2.12.19 (prod, win32), so it's not specific to the latest beta, which I'm on now.
I disabled any custom css, and still experience this issue. It seems this grey bar 'hides', or makes not visible the cursor, when the cursor is at the left edge of the editor pane (the cursor behaves normally, it's just not visible when at that left edge).
Rich Markdown and macOS theme plugins disabled: grey bar goes away, invisible cursor at left edge remains
Rich Markdown disabled: grey bar persists, invisible cursor at left edge remains
macOS disabled: grey bar goes away, invisible cursor at left edge remains
So it looks like the grey bar is related to macOS plugin, and NOT related to the invisible cursor at left edge. So perhaps I should mark this grey bar issue as resolved and focus on the invisible cursor issue.