[FIX] web_editor: restore overlay top options on top of the page

When the overlay covers a snippet, if that snippet is at the top of
the page, the options at the top of the overlay are hidden. A system
was made to make them change position in that case but this was broken
by [1], leaving the toggling of a class for that case but without its
related scss.

This commit restores that scss and also improves the detection of that
problematic case.

[1]: https://github.com/odoo/odoo/commit/4f27e52cabb77b8b1a9637a11185ddf882adc9af



closes odoo/odoo#60464

X-original-commit: 97f11571f9e3deacc9b9e1dc0d2cc8fc6b93f15c
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>