That's why they're now taking to tracking the PLA aircraft with ground-based sites. I don't know if that means they won't scramble until they know the invasion is coming or if they'll do it but less frequently.
A better question is whether China will attempt to invade before the F-35 could reasonably be made available if a sale was approved during the Biden administration.
I don't think any invasion would be predicated on a F-35 sale. It is not a "wunderwaffe", and the price precludes any shift of the strategic balance. Honestly, there are probably enough intelligence assets in Taiwan that such a sale would be cautiously welcomed behind closed doors.
My point is more to the fact that there is no obvious path forward at the moment. IDF is getting old (needs some major upgrades like AESA, but the engines are probably too weak for it), Mirage is a dead end, ~100 F-16V is a fraction of the front line force in the early 2000's (~300 between Mirage, IDF, F-16A/B).
My question then, if China is waiting for Taiwan to slip up why warn the US? Let the trip happen, Kelly Craft announces US supports independent Taiwan for UN or whatever, then declare war immediately after.
I don't think that the idea is to fight a war over Taiwan, just to be able to win one. PRC was willing to wait 70 years for HK, they are willing to wait for TW. Without a term limit, Xi Jinping can afford to wait. The time limitation is really more on America's watch, not PRC's. In another 10 years, PLAN will likely be able to float 2 CVBGs to the west coast of the island. At that point, the strategic balance is probably sealed. That isn't to say that the government can't stubbornly hold out and ignore reality (see: Ukraine), but it would be in the best interest to figure out a solution that is win-win.