Re: [editing-explainer] What browser internals can be exposed to help us move the caret in (and against) the block direction? (#56)

@rniwa 
> Doesn't that mean we just need something like "beforeselectionchange" event that exposes where the selection will be and let the website override the new selection as needed?

Consider the following use case. 
```
verrrry looooong liiineeee
short line
verrrry looooong liiin|eeee
```

The caret is marked as a | (pipe) symbol. When the user presses the UP key twice, the caret temporarily moves to the end of the short line and then back to the same X axis coordinate on the top line:  
```
verrrry looooong liiin|eeee
short line
verrrry looooong liiineeee
```

So, when the user moves UP/DOWN, we need to override the UA selection but keep this magic X coordinate. However, when moving LEFT/RIGHT both the selection and magic X coordinate should be overridden.

Another examples of hidden selection state are:
 - the pending click count. On some platforms double click means select a word, triple click means select the whole paragraph.
 - whether the magnifying glass is shown on iOS or not
However, these two examples fall more into the mouse triggered selection changes.




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