Slow highlighting on KDevelop start with many restored documents

[David Nolden]

All open documents are parsed+highlighted fully. We don't
differentiate between the active one and other ones.

What we could do is:
a) Changing the background-parser so that it processes the currently
active document first
or
b) Add the currently active document with a higher priority to the
background parser (maybe better)

But we somehow need to track the currently active document then.

Greetings, David