here If you buy ExtJS 4 and ExtJS 5 comes out in the next 12 months, you get it for free.
Chromium manages it by having extremely rich automated testing and mandating that code must have passed the full set of automated tests on all platforms and have passed a code review cycle before it is checked in. They have their "try" server infrastructure in place to enable developers to run automated tests before committing code. For some projects realizing this is nothing short of herculean, and even if you have solid automated testing in place there's a lot of tooling to be developed, preciously little of which you can get off-the-shelf. If it interests you, they have a slide deck explaining how to work on the chromium codebase.
And I wonder about backward compatibility of Extjs 5. Will there be as many changes between 5 vs 4 as with 4 vs 3?
I'm considering Sencha Complete which provides future Ext 5, but I wouldn't like to convert my projects to Ext 4 now, and again later to Ext 5.
Can some member of the development team tell us what Ext 5 is supposed to look like in regard to compatibility?
Ext JS 5 will not be as drastic of a change as Ext JS 4 was. We needed to make the drastic changes in Ext JS 4 to become a better framework so Ext JS 5 will build on top of this.
Do you have an estimate when we could see Ext JS 5 beta? ...still this year or Q1 2014?
For us, going from 3-4 was painful but in the long run worth it. Our code is much more readable and 4.2.2 has great speed improvements for trees and grids vs prior 4.x. It's almost as fast as 3.x. The big leap was implementing the "class" system. That's where most of our work was focused.