Both flexbox and grid layouts need the container div to be the direct
parent of the children it lays out. To enable that, we need a class that
can select the direct parent of the list widget in PageTemplate.tid so
that that class can have `display: flex` or `display: grid` applied to
it. The `tc-page-container` div is not suitable, because it contains
a `<$dropzone>` inside it, and the dropzone widget creates a div so
tc-page-container is no longer the direct parent of the list. Instead,
a tc-page-container-inner class is added to the dropzone widget in
addition to its existing tc-dropzone class, so that grid or flexbox
layouts can target tc-page-container-inner for setting the appropriate
CSS `display` property.
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