On 11/21/20 1:29 AM Greg Ercolano wrote:
> On 2020-11-20 15:22, Albrecht Schlosser wrote:
>>
>> I started with Ian's code and tried my idea but it doesn't seem to work
>> as requested. Maybe the position and size of this invisible box (box_i)
>> can be changed to get the correct relations but I don't know. Would need
>> to look into the resizing code to see how the relations are calculated
>> (but not tonight).

I couldn't resist and did it...
;-
>> See attached file r.cxx.
>
>     Thanks -- yes, I just don't think it's possible to keep B centered doing any of the resizable
>     tricks we're normally used to.

Well, it *is* possible! :-)

See attached new version of 'r.cxx'. Running it and resizing makes the
layout look familiar - like test/cube.cxx which I know was the reason
you asked. ;-)

The "secret" (revealed by a little math and the code in
Fl_Group::resize()) is to put the invisible box centered over the border
between the two boxes B and C. As long as the overlap is equal on both
sides the two boxes (ab_grp and box_c) resize by the same amount and
therefore A and C resize symmetrically.

For simplicity I made all the boxes adjacent with widths 200, 100, 200,
resp. and put the invisible box on x = 250 with width = 100 (50 left of
the border and 50 right of the border). That's it!