Lazarus 1.6RC2 & FPC 3.0.0 on Raspbian Jessie

When I try to change the horizontal width of the code editor window in Lazarus I am able to increase it but not to reduce it. I made a screenshot image so I could evaluate the width in pixels and it seems like it refuses to reduce smaller than 1060 pixels or so. I can grab the lower right corner handle but I cannot move it left; up/down/right is OK though.
Likewise I can grab the right border handle and move it to the right but not left.
If I increase the width and then try to reduce it I can do it down to about 1060 pixels.
The same exact behaviour on the HDMI output channel (display :0) and on the VNC connection (display :1)

What can be the reason for this behaviour?