Bad performance with two linked nodes far away from each other

[l3td33r]

Steps to reproduce:

1. Create two linked nodes, far away from each other (the greater the distance, the greater the effect)
2. Zoom into one of them, so that the second one disappears from screen
3. Performance gradually becomes worse, the more you zoom

[juzzlin]

This is a problem in Qt. Not sure what we could do to improve performance. Maybe switch off shadow effects.

[l3td33r]

If the bug is related to drawing lines offscreen, then how about drawing fake lines, from the node to the approximated position on the screen edge? Instead of the actual lines.

[juzzlin]

Well, that could indeed work.

[juzzlin]

This seems to work much better on Windows for some reason, at least on Qt 5.15.2.

[juzzlin]

This seems to be somewhat fixed in 4.1.0 when the shadow effect optimization is enabled.

[juzzlin]

Actually, zooming can still be improved so I'll leave this open.