SUMMARY  [This is my best guess at product/component.]

This issue is minor; but aggravating.  Three of four window resize grab handles are off-screen when a window is against a screen edge.  Only the top grab handle is available.

When a new window opens with one or more edges against a screen edge, the resize handle is not available because the handle's grab area is just off screen. This is true for left, bottom and right; top handle can be grabbed when window edge is at screen top.

STEPS TO REPRODUCE
1. open any window
2. if it is against left, bottom or right screen edges, the window cannot be resized from that edge. (Bottom issue is obvious on non-primary screen; on main screen, bottom edge against taskbar often results in tooltips popping up)
3. 

OBSERVED RESULT  [Below was originally written using fixed-width font.]
The "grab" area is just outside the window geometry. Maximized was referenced only to say that is not part of the issue:
     - move the window if not maximized
     - if maximized, restore and likely still have to move the window
So, when the window edge aligns with the screen edge, the hot zone is outside the screen area and not available.

Maybe this was not clear enough, or I should have avoided mentioning anything about maximized...  Let me try a graphic representation:

Using left screen edge as representative for left, bottom & right... 

Window not quite at edge:
        ||
        <o>   Cursor changes to 'grab'||
        ||

Window at edge:
        |
        |\    Cursor can't reach grab's "hot" area
        |

Greatly expanded:
                  | grab  |<== window edge    
    screen edge==>| area  |   window almost at edge
                  | ~10px |
                  |  <o>  |  <== cursor changes from \ to <o>
                  |     <=|=== grab area within which cursor changes
                  |       |
    screen edge==>|<== window edge 
          | grab  |   
          | area  |   window AT edge
          | ~10px |
          |     <=|=== grab area within which cursor can't go

Also, when trying to hit the grab area, user has to consciously move cursor *outside* the window boundaries which is non-intuitive.

The "hot" zone should straddle/be centered over the window edge:
screen edge==>|<== window edge 
          | gr|ab |   
          | ar|ea |   window AT edge
          |~10|px |
          |   |   |
Now, the hot zone is available and intuitive.

HTH
The only workarounds are:
- move the window if not maximized
- if maximized, restore and likely still have to move the window

EXPECTED RESULT

Rather than being being just outside, the grab handles should be just inside the window edges.

SOFTWARE/OS VERSIONS
Operating System: Mageia 9
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.88.0
Qt Version: 5.15.2
Kernel Version: 5.16.14-server-1.mga9 (64-bit)
Graphics Platform: X11
Processors: 20 × 12th Gen Intel® Core™ i7-12700K
Memory: 62.5 GiB of RAM
Graphics Processor: AMD Radeon RX 6600 XT


ADDITIONAL INFORMATION
See also https://bugs.mageia.org/show_bug.cgi?id=30154