This is happening because you have created a component U1 which includes the board outline, the 2x20 way connector and the 4 mounting holes.

• It is not clear why you have done this;
• It is not good practice to do this.

If instead you:

1. Create the board outline;
2. Place the holes;
3. Find or create a PCB Lib for the 2x20 way connector;
4. Find and place the other connectors and U2;
5. Route the PCB;

then the flashing as you pass over the nets will not happen.

You may be able to fix your existing PCB by selecting U1, Ungrouping it and then regrouping just the elements making up the 2x20 way connector. Beware however that you will find that you have several small track segments that you should delete before you group the connector:

and then retrack after regrouping the elements for the connector.

To avoid odd behaviour like this, please follow the recommended Design Flow as described in the documentation in (2) in:

https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a

• Even for what seems like a trivial PCB, starting with a properly constructed schematic which is then converted to a PCB, saves all sorts of trouble.

1. It is not clear why you have done this;
This is an older component and probably not something I would do again but it does mean that you skip steps 1-3 for each new design it is based on.  The time saved, in this particular component is not insignificant.  The only alternative I am aware of would be to create a project with a schematic and PCB and clone for each new design.

2. It is not good practice to do this.
Won't disagree with you there.

Instead of making a your outline, connector and holes into a PCB Footprint, you could make it as a pcb but then save it as a (reusable) PCB Module.

at 6.3, EasyEDA will provide a option to disable this feature.

@UserSupport,

That will help in cases where there are lots of tracks on each layer but in this particular case it may not help because EasyEDA is seeing the whole board outline, connector and holes as a single PCB Footprint so it lights up as soon as the mouse hovers inside it.