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.
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).
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?