Minimum window size
Windows have a minimum size, and can't (obviously) be resized below these specifications.
If you are on X (not Wayland) You can see what is the minimum width & hight of a window* : open a terminal and type the command:
~$ xprop WM_NORMAL_HINTS
Then press Return. Subsequently, click on the target window.
The output will look like:
program specified minimum size: 551 by 159
program specified base size: 0 by 0
window gravity: NorthWest
In the case of Codeblocks, it turns out the window's minwidth is at least the width of its menu bar.
*For the exact size & position of a window, we also need to take into account the _NET_FRAME_EXTENTS and _GTK_FRAME_EXTENTS