coordinate track extension

[Ichthyostega]

Coordinate the vertical size allocation between the track header pane and the track body display.
Implementing this requires a global »Display Evaluation Pass« over the timeline structure as a whole.

[Ichthyostega]

We have to care for two distinct concerns here:

• ensure the track content area is sufficiently sized to hold the actual content (clips)
• keep the Track-Header size requirement and the Body size requirement in sync, possibly transporting an offset downwards

[Ichthyostega]

Meanwhile, a implementation was designed and mostly coded; however, validating this design was hampered by the missing implementation for the actual Clip widget #1038 and a first draft of the Timeline header pane layout #1018; these would have to serve as active counterparts in the header as well as in the content area, to demonstrate that the layout negotiation actually works as designed

[Ichthyostega]

While integrating the draft for Zoom handling, it became clear that there is still a conceptional shortcoming with respect to nested tracks, leading to "run-away" of the layout as soon as the size of the window is changed. This became obvious now, since the ZoomWindow actually triggers re-evaluation of the complete layout, while up to now the layout was only calculated once, at start.

Seemingly this is related to the yet unsolved question how to deal with collapsed tracks (see #1265) and how to handle content in group tracks (see #1266) — something in the header structure must be added to accommodate for the additional space showing up at nested scopes

[Ichthyostega <prg@…>]

In 25be54ec/Lumiera:

Timeline: remove diagnostic code (closes: #1211)

Layout calculation and balancing between body and track header
now works reasonably well, labels are placed properly and the
calculated layout remains stable when changing window size,
connected panes and scrollbars behave as expected now.

Quite a feat!

[Undercover Agent]

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