but this was temporary for me. I have since been able to resize the editors whatever the layout.
Try resizing the editors horizontally (if you have the Matchboard or some other tab docked to the right).
I should have specified that I'm encountering this problem when I try to resize the editor horizzontally.
But, I've just noticed that it's sufficient to resize just a little bit the general TB pane (which in my case is docked on the editor right side) to have source and target automatically resize at 50%-50%. That is, I cannot choose the resizing ratio. Then, if I resize the TB pane again (even just a little bit), everything goes back to the condition shown in the picture above.
In other words, the editor gets resized only by resizing the right-most pane.
In my new Linux installation I can't adjust the source-target relative length with the slider. Now it's permanently like this:
In Windows the slider works normally. Might this be related to the fact that the source segment is Japanese?
Jean, how about your Linux installation?