This may be outside our control. There is a selection highlight around the window when it has focus if that helps.

Focus is automatically changed when the user clicks in a window. But we can explicitly give the focus to a window.

Either call the focus command with a tk window id:

focus .vtkSlicerWindow3.vtkKWFrame3.vtkKWSplitFrame0.vtkKWFrame4.vtkKWRenderWidget2.vtkKWCoreWidget0

Or call the Focus method of a KWWidget object:

[$::slicer3::ApplicationGUI GetMainSlicerWindow] Focus

Hmm, I suppose we could do that after every interaction with the SliceControllerWidget.

I know in the past someone tried a mode where we called focus on ever Enter event, but on windows that caused the window to raise as well, and that was annoying.

Generally the focus handling is OK, only the slice browsing event handling is problematic.

I guess this part of the GUI will be completely reworked during the QT migration, so I would propose not to rework anything just add an explicit focus change in vtkSlicerSliceControllerWidget::ProcessWidgetEvents when the vtkKWScale::ScaleValueStartChangingEvent is received.

I don't think it could cause any issue, as the same method is called whenever the user clicks on the image:
void vtkKWRenderWidget::MouseButtonPressCallback(...)
{
this->VTKWidget->Focus();
...

I've done a very simple implementation to show what I mean (and it actually works for me, but the code has to be cleaned up the "find the sliceGUI for this controller" should be moved to a separate method, pointer checks, comments are needed). See the attached patch file.

Integrated a fix into latest Slicer trunk revision 12261.

Problem 1 (set focus if slice selector slider is clicked) is solved. Problem 2 would have needed many modifications (the focus should have been set whenever the user switches to any a single-view layout or switches between tabs in tabbed slice or 3D view layouts), that is significant effort and limited benefit (fix for problem 1 already eliminates most of the issues, and the KWWidget implementation is being phased out anyway).

The problem can be closed now.