rear chain adjustment has nothing to do with the transmission jumping out. Deluth's comment , suggestion to adjust the shifter to a different position, is correct.
What happens when the trans "jumps out", is the force of power is pushing against the dogs, which usually are engaged, and the spring loaded drum is held in position, isn't in place.
If the transmission is not engaged fully, the power, pushed the dogs out of the engagement, with time, repeatedly letting this happen, 'rounds off' the corners of the dogs, and prematurely wears the gears dogs.
Then, instead of having flats on the dogs to transfer power, the dogs are now rounded, and do not want to fully engage.
If when shifting, you find you shift twice, and it stays in the same gear, on the second shift, this is telling you the last gear wasn't engaged fully, not allowing the next to engage.
This if often caused by the rider.
Since the transmission is constant mesh, it will withstand strong force in shifts easily, and should be shifted with forceful shifts.
A shifter lever, not adjusted, or a lazy foot, not moving all the way, is easily adjusted.
Using tennis shoes, sandals, or sunday shoes with riding, "Shifting hurts my foot", is also a recipe for ruining the transmission.
The stock transmission, will handle strong shifting. Few ever fully understand this, and its common to hear "my bike pops out of gear".
comments like this, indicate, the rider isn't shifting into the next gear fully. Baby'ing the shifts, either because of misunderstanding the force needed to shift properly, or timidly trying to "be nice" to the transmission.
both are incorrect and improper techniques when shifting a constant mesh transmission.
As Deluth mentioned, adjusting the lever down, so as to have more force on the upshift, and or, using a strong foot movement when shifting, will make for good shifts.
Also, using the clutch to fully disengage the power to allow full movement of the transmission movement, only takes a fraction of a second,
but like the foot movement, this needs to done quickly, forcefully, and in synchronization, together.
When I read, about the masses who replace the stock fully long lever, with a shorty, seldom do they then use the clutch fully.
This is asking for disaster.
Once the transmission has "popped out of gear" a dozen or so times, or "it only happens in one gear", its already too late. the dogs are toast. Transmission rebuild time.
Rowing the box when new, can help "break in" a new transmission.
When I mentioned this in previous posts, I was mocked for this concept.
The drive chain, slack or tight, has no effect on transmission shifts. Its your foot movement to forcefully put the trans fully into the next gear, your hand movement to dis-engage the clutch allowing this to happen, and not doing this is basically not knowing how to shift. And that's a fact Jack. ...cue Mr. Nice...