Text selection switches between partial and multi-select when cursor is between blocks

[jasmussen]

When making a partial selection between two blocks, and the selection is made somewhat quickly, the blue selection borders do not appear. However when moving the cursor slowly and pausing in the space between blocks, the blue borders do appear:

When you are making a partial selection, is-partially-selected and is-multi-selected classes are both applied to the block. However when your cursor is between two blocks, is-partially-selected is briefly removed, as shown in this GIF of the inspector and the applied classes during selection:

Ideally the is-partially-selected class should remain in place even when hovering between blocks, up until the point where when a non-text block is selected and the partial selection becomes a multi-selection. This would fix the flickering issue you can otherwise see when selecting multiple paragraphs.

See also: #44148.

[jasmussen]

Created #44154 as a potential fix for this one.