Fix horizontal scrollbar on full-wide blocks with nesting.

[jasmussen]

This fixes #15192.

This PR fixes two issues:

1. There was an issue, probably rebase related, where the columns block had CSS to prevent horizontal scrollbars when fullwide, but which didn't work anymore. I simply fixed the selector again.
2. The recent merge of the clickthrough PR failed to take into account fullwide blocks with nesting, and caused a horizontal scrollbar due to the overlay extending beyond the canvas.

To test this PR, please verify that full wide alignments work as intended. You can test columns, image, embed, media & text.

Please also verify that blocks with nesting work as intended, both in fullwide and not wide situations.

Before:

After:

[kjellr]

Works well on my end. Thanks for fixing this, @jasmussen!

Before

After

Note that this does reinstate the extra left/right margins for columns. We may end up wanting to go back and revert/update this change for Twenty Nineteen. It turns out this change was prompted by the bug that's fixed here. 😄 https://core.trac.wordpress.org/ticket/46643

Revising that approach would eliminate this bug:
https://core.trac.wordpress.org/ticket/47044

[jasmussen]

Note that this does reinstate the extra left/right margins for columns. We may end up wanting to go back and revert/update this change for Twenty Nineteen. It turns out this change was prompted by the bug that's fixed here. 😄 https://core.trac.wordpress.org/ticket/46643

I'm sorry that I didn't fix this before so that TwentyNineteen aligned i the wrong direction.

The reason why there is extra margin is actually in absence of a better solution. As is noted also in the code comment:

// Fullwide: show margin left/right to ensure there's room for the side UI.
// This is not a 1:1 preview with the front-end where these margins would presumably be zero.

In other words — we should look at a better solution, but do that holistically. Perhaps with the recent improvements to selecting nested blocks, and your incoming outlines PR, maybe now we have the tools to do so? Maybe in a way that's the same for fullwide images and fullwide columns?

In the mean time, I don't have a strong opinion on changing TwentyNineteen. If we have a great idea for how to improve this in Gutenberg and believe we can ship it faster, maybe it's fine?

Thanks for the review!

[kjellr]

In the mean time, I don't have a strong opinion on changing TwentyNineteen. If we have a great idea for how to improve this in Gutenberg and believe we can ship it faster, maybe it's fine?

Agreed. Let's leave for now. Once the nesting stuff gets finalized we can re-assess the extra padding we currently have on columns. 👍