[Lazarus] TSplitter refuses to move to the left with mouse (but moves via code)

[Jürgen Hestermann]

I have a TSplitter that separates elements on the left from those on the right.
I use code to move this splitter to the left or right in an KeyDown routine.
Example:

-----------------------------------------------------------------------------------
case Key of VK_LEFT :
    begin
    if Shift=[ssShift] then
       begin
       if SplitterLinks.Left>50 then
          begin
          SplitterLinks.Left := SplitterLinks.Left-5;
          end;
    Key := 0;
    end;
-----------------------------------------------------------------------------------

This works fine for moving to the left and to the right.

But when I try to move the splitter with the mouse,
only moving to the right works okay.
Moving to the left only moves the splitter 1 pixel (I think).
Moving it again moves it another 1 pixel and so on.
But I cannot move it multiple pixels in one step (as I can when
moving the right).

Is this a bug?

The splitter is not anchored to the left or right (only top and bottom).
Instead, elements on the left and right are anchored to the splitter.