Fixed bokeh LayoutPlot bugs

[philippjfr]

Turns out that the bokeh LayoutPlot implementation was wrong in all kinds of subtle ways so I've completely rewritten the implementation from scratch. The main issue was that AdjointLayout plots were not correctly positioned on the grid. Secondly when enabling tabs AdjointLayout plots were split across multiple tabs.

• Fixes cols not lining up in bokeh #1489
• Fixes Layout Tab Bug - Variable Lengths Create None Tabs #2568
• Fixes AdjointLayout does not layout properly in a multi-row Layout #2652
• Adds unit test

[philippjfr]

Ready to review and merge. Also just as a quick note, while I didn't add many new unit tests here there are a number of unit tests for bokeh layouts which are unchanged. Additionally I wrote a little snippet to generate random layouts with combinations of elements and AdjointLayouts, which I tested heavily (must have run it several hundred times to confirm things were working).

[jlstevens]

Turns out that the bokeh LayoutPlot implementation was wrong in all kinds of subtle ways...

Why do I feel deja vu? ;-p

To be fair it has always been very tricky code and I am always happy to see it refactored and improved. I've had to restart a test build for what I assume was a transient but otherwise I am happy to merge.

The only thing I would ask you to consider is whether there is another unit test or two you could add - the more unit tests the better given how tricky it is to get right!

[philippjfr]

Why do I feel deja vu? ;-p

I think this code has been around since I first wrote the bokeh backend, so it has actually held up surprisingly well, unlike the matplotlib version.

The only thing I would ask you to consider is whether there is another unit test or two you could add. The more unit tests the better given how tricky it is to get right!

Added a unit test for the tabs issue, layouts themselves have enough unit tests covering them.

[jlstevens]

Added a unit test for the tabs issue, layouts themselves have enough unit tests covering them.

Thanks. I'll merge when the tests go green.

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