GUI unresponsive during IBD

[rebroad]

I thought there was an issue raised for this already but it appears not. It's an issue in 0.21 where the GUI doesn't respond to mouse clicks until ActivateBestChain() has finished.

This is similar to #247 but it's not sticking at "done loading" but the main core window is freezing when blocks are being validated.

[hebasto]

This is similar to #247 but it's not sticking at "done loading" but the main core window is freezing when blocks are being validated.

I verified how ActivateBestChain affects on the GUI by inserting explicit delays before each return statement. No GUI unresponsiveness observed except for the case described in #247.

[hebasto]

From bitcoin/bitcoin#29293:

Seems that "connecting to peers" and the beginning of "synchronizing with network" operation block UI thread.