ALT key function and GIMP

one of the features of Linux seems that you can press the ALT key and move the application window without need to click on the window-title. This might be cool, but I want to make basic image manipulations with GIMP and need the ALT key in the application to move objects. Can I disable the system ALT feature somehow?
Best Regards, Andreas

Why you need alt key for that?
You can just select Move Tool and drag layers where you want.

I have a simple image with a green object. I use select by color to select this object. If I want to move the selected content I need to press ALT+Shift key. But this triggers the window manager (regardless that I also press the Shift key) to move the application window. Without keys I move the selection border (which makes no sense for me). (BTW: This was the opposite in GIMP2.4 - no idea why they changed it) When I use Google I see this seems a long dispute between GIMP and window manger about who owns this key.

I’m using GIMP since 20 years and never have any problems with moving layers selections and so on.
Seems to me like you about to complicate simple things, but you can configure Window Manager as you want by editing ~/.config/openbox/rc.xml.
Documentation may be helpful → Help:Contents - Openbox

PS. If you like to move what is inside selection you probably need to make new layer from selection. In recent GIMP releases they called this Float shift + ctrl + L

The basic ALT+Mouse functions can be switched out in rc.xml Actions between the first and last line here are inactive. (It’s not easy to insert and escape these
four characters here)

      <mousebind action="Press" button="A-Left">
        <action name="Focus"/>
        <action name="Raise"/>
      <mousebind action="Click" button="A-Left">
        <action name="Unshade"/>
      <mousebind action="Drag" button="A-Left">
        <action name="Move"/>
      <mousebind action="Press" button="A-Right">
        <action name="Focus"/>
        <action name="Raise"/>
        <action name="Unshade"/>
      <mousebind action="Drag" button="A-Right">
        <action name="Resize"/>
      <mousebind action="Press" button="A-Middle">
        <action name="Lower"/>
        <action name="FocusToBottom"/>
        <action name="Unfocus"/>

Thanks, yes maybe I am not working ‘correct’ with GIMP. I just saw this manual 2. Creating and Using Selections
and 7.6 and 7.7 are showing what I want to do. But this did not work (for me). Interestingly also booting Win10 and using GIMP portable now, this function also does not work, and there is no similar ALT functionality in the window manager of W10. Thanks zolw for the hint to rc.xml, maybe I give that a short try. I was also able to cut the selection move the outline and paste the content at different location - maybe thats enough.
CTRL+Shift+L is also working when I select layer in tool adjustments. I’ll get closer…