TSynGutterLineOverview = class(TSynGutterPartBase)
private
FProviders: TSynGutterLineOverviewProviderList;
FWinControl: TSynChildWinControl;
....
procedure TSynGutterLineOverview.Init;
begin
...
FWinControl := TSynChildWinControl.Create(Self);
That will need to override MouseDown/Up etc, and forward it.
Or maybe it can be made to ignore them in the WindProc handler, and pass them to parent.
The problem is that the overviewgutter invalidates at a different y pos than the rest of SynEdit.
In cases where the minimum bounding rect will be invalidated/painted this leads to lots of extra painting (especially during scroll, and that can visible slow down scrolling).
So this is only needed for painting. All else (hittest, cursor, mouse, etc) can go to SynEdit.