It seems like it's a very common requirement to select text in a grid. I know my users demand it. There is an override for it in the grid faq for Ext 3. I just spent the last half hour or so trying to apply it to 4. It's similar, but now I have to override both View and TableChunker.
It was a good experience and I learned some new things about 4, but now I'm asking myself why. It seems like this long standing workaround deserves a new feature. Since you're so radically, redesigning the grids anyway, could you just give us a config option to make the text selectable and save everyone the hassle of having to find the overrides?
The same can be said for some of the other long standing grid cosmetic issues - letting lines wrap, for example, could be a config option.