Customizer: gap above widget section when focusing from preview shift+click

[mattwiebe]

When focusing on a widget via shift+click in the preview iframe, the widget area panel will render with an incorrect margin-top, producing a large, ugly space.

Steps to reproduce:

1. Open the Customizer
2. Shift + click on any widget area
3. See a large gap above the widget's widget area section

Note that this stops happening every time after you've been navigating through other sections/panels in the Customizer. Focusing on a widget first produces reliable results. Screenshot and patch incoming.

[mattwiebe]

All the diff does is move section._recalculateTopMargin() into a setTimeout call with a 0ms timeout to bump it to the top of the call stack, since it's otherwise making calculations based on not quite current layout.

[ocean90]

Related: #34391

[westonruter]

I've been noticing this increasingly as well. It seems to be specifically when focusing on a section (or a control within a section) that is within a panel. It also seems to only happen when the contained panel was previously collapsed. If you collapse the section and re-expand it, subsequent focuses on other sections within that panel seem to not have the glitch.

[westonruter]

In 36805:

Customize: Remove gap above section contained in panel when initially focusing on control.

Props mattwiebe.
Fixes #36050.