KB: Indicate visual cues at the coordinate limit of PCB design space beyond which objects should not be placed
Altium Designer
[Why] Objects cannot be moved (or selected by mouse if already placed, say, from mechanical dxf import) at some extreme coordinates beyond PCB design space limit [What] The PCB design space is bounded to the coordinates between (0,0) and (100000mil,100000mil) or 100x100 inches or 2540x2540 mm. No objects are allowed beyond this bounding box. [How] Placing a horizontal/vertical guides (yellow line) through the absolute origin would serve as visual cues thereafter indicating anything to the bottom/left of these guide lines need to be moved upward or to the right. Since you will not be able to select these objects already outside the limit merely by a mouse click, you will have to use PCB List panel to locate them and type valid positive coordinates.
Solution Details
Altium restricts component placement and routing to the upper-right positive quadrant up to (100000mil,100000mil).https://www.altium.com/documentation/altium-designer/pcb-grids-system#:~:text=the%20extreme%20lower%2Dleft%20of%20the%20100%20x%20100%20inch%20design%20space
Some objects such as during dxf import, however, can end up outside this bounding limit.
To determine the current location of the absolute origin, you can use Edit ► Jump ► Absolute Origin, your cursor will jump to the original Absolute Origin. Everything you do should be above and to the right of this point.
To reset the Origin, use Edit ► Origin ► Reset. Then you can move all your current work into the upper right quadrant by using Ctrl+A to select everything, then use Edit ► Move ► Move Selection to get a cross-hair pointer, use this to click a reference point and then move the cross-hair to where you would like the reference point to go.
Alternatively, if you need to move these objects already outside the limit one by one exclusively, since you will not be able to do so by click&drag mouse operation, you will have to use PCB List panel to locate them and type valid positive coordinates.
https://www.altium.com/documentation/altium-designer/editing-multiple-pcb-design-objects#!pcb_list_panel
Then, after your board is well into the upper right quadrant, you can use the Edit ► Origin ► Set command to place a relative origin wherever it works best for you as your relative (0,0) location.
https://www.altium.com/documentation/altium-designer/pcb-grids-system#!setting_the_board_origin
Placing a horizontal/vertical guides through the absolute origin (Place » Work Guides » Place Horizontal/Vertical Guide, followed by keyboard shortcut J and A to jump to the origin) would serve as visual cues thereafter indicating anything to the bottom/left of these guide lines need to be moved upward or to the right.
https://www.altium.com/documentation/altium-designer/pcb-working-with-grids-guides#!snap-guides
If guides are not needed, they can be disabled or deleted in Properties panel, under Guide Manager section. Guide lines (and grid lines) will only show if the box is checked (default) for "Show Grid" in the General Settings region on the View Options tab of the View Configuration panel.