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Created attachment 149611 [details]
the file

I probably missed one step.

Zoom related, it seems.

Good catch, seems the evaluation of the panes sizes does not consider the zoom level.

Fixed.

I am still seeing the vertical split line a few pixels too far to the left,
i.e., if I colour D1 red, the red is visible to the right of the split.

I'd say one pixel to the left.

Created attachment 149639 [details]
Screenshot

Two pixels, both directions.

Created attachment 149653 [details] [review]
partial fix

At least the panes separators look now at the right place. At 111% the grid and the cursor are still misplaced (looks good in RTL mode, btw).

Looks good -- please commit.

At, say, 200% we are pixel perfect.  At 106% (this file), we are off by
one pixel when we have multiple panes.

Comment on attachment 149653 [details] [review]
partial fix

Still need to address the grid and cursor position issues.

the one pixel offset seems now fixed.