Qt application output:

First screenshot (moved cursor in top-left corner, triggering "Show all windows" in Plasma)
WinId: 29526304 (0x1c28920)
QXcbConnection: XCB error: 2 (BadValue), sequence: 882, resource id: 0, major code: 53 (CreatePixmap), minor code: 0
QXcbConnection: XCB error: 9 (BadDrawable), sequence: 883, resource id: 10485785, major code: 55 (CreateGC), minor code: 0
QXcbConnection: XCB error: 9 (BadDrawable), sequence: 884, resource id: 10485785, major code: 62 (CopyArea), minor code: 0
QXcbConnection: XCB error: 9 (BadDrawable), sequence: 885, resource id: 10485785, major code: 73 (GetImage), minor code: 0
QXcbConnection: XCB error: 13 (BadGC), sequence: 886, resource id: 10485785, major code: 60 (FreeGC), minor code: 0
QXcbConnection: XCB error: 4 (BadPixmap), sequence: 887, resource id: 10485785, major code: 54 (FreePixmap), minor code: 0

Second screenshot (normal window):
WinId: 2097277 (0x20007d)
resulting pixmap:
QPixmap(QSize(1276, 691),depth=32,devicePixelRatio=2,cacheKey=0xd6700000002)

xscope log:

 5.79: Client (pid 5555 spectacle) -->   24 bytes
         ............REQUEST: InternAtom
              only-if-exists: False
                        name: "WM_STATE"
         ............REQUEST: QueryPointer
                      window: WIN 00000148
 5.79:                                    64 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: InternAtom
                                                        atom: <WM_STATE>
                                         ..............REPLY: QueryPointer
                                                 same-screen: True
                                                        root: WIN 00000148
                                                       child: WIN 01c28920
                                                      root-x: 1
                                                      root-y: 0
                                                       win-x: 1
                                                       win-y: 0
                                                        mask: Mod2
 5.79: Client (pid 5555 spectacle) -->   24 bytes
         ............REQUEST: GetProperty
                      delete: False
                      window: WIN 01c28920
                    property: <WM_STATE>
                        type: AnyPropertyType
                 long-offset: 00000000
 5.79:                                    32 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: GetProperty
                                                      format: 00
                                                        type: <NONE>
                                                 bytes-after: 00000000
 5.79: Client (pid 5555 spectacle) -->    8 bytes
         ............REQUEST: QueryTree
                      window: WIN 01c28920
 5.79:                                    32 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: QueryTree
                                                        root: WIN 00000148
                                                      parent: WIN 00000148
 5.79: Client (pid 5555 spectacle) -->   96 bytes
         ............REQUEST: GetProperty
                      delete: False
                      window: WIN 01c28920
                    property: <WM_STATE>
                        type: <WM_STATE>
                 long-offset: 00000000
         ............REQUEST: GetProperty
                      delete: False
                      window: WIN 01c28920
                    property: <_NET_WM_NAME>
                        type: <UTF8_STRING>
                 long-offset: 00000000
         ............REQUEST: GetProperty
                      delete: False
                      window: WIN 01c28920
                    property: <WM_TRANSIENT_FOR>
                        type: <WINDOW>
                 long-offset: 00000000
         ............REQUEST: GetProperty
                      delete: False
                      window: WIN 01c28920
                    property: <WM_CLASS>
                        type: <STRING>
                 long-offset: 00000000
 5.79:                                   128 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: GetProperty
                                                      format: 00
                                                        type: <NONE>
                                                 bytes-after: 00000000
                                         ..............REPLY: GetProperty
                                                      format: 00
                                                        type: <NONE>
                                                 bytes-after: 00000000
                                         ..............REPLY: GetProperty
                                                      format: 00
                                                        type: <NONE>
                                                 bytes-after: 00000000
                                         ..............REPLY: GetProperty
                                                      format: 00
                                                        type: <NONE>
                                                 bytes-after: 00000000
 5.79: Client (pid 5555 spectacle) -->   24 bytes
         ............REQUEST: GetProperty
                      delete: False
                      window: WIN 01c28920
                    property: <WM_NAME>
                        type: AnyPropertyType
                 long-offset: 00000000
 5.79:                                    32 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: GetProperty
                                                      format: 00
                                                        type: <NONE>
                                                 bytes-after: 00000000
 5.79: Client (pid 5555 spectacle) -->   24 bytes
         ............REQUEST: InternAtom
              only-if-exists: False
                        name: "WM_STATE"
         ............REQUEST: QueryPointer
                      window: WIN 00000148
 5.79:                                    64 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: InternAtom
                                                        atom: <WM_STATE>
                                         ..............REPLY: QueryPointer
                                                 same-screen: True
                                                        root: WIN 00000148
                                                       child: WIN 01c28920
                                                      root-x: 1
                                                      root-y: 0
                                                       win-x: 1
                                                       win-y: 0
                                                        mask: Mod2
 5.79: Client (pid 5555 spectacle) -->   24 bytes
         ............REQUEST: GetProperty
                      delete: False
                      window: WIN 01c28920
                    property: <WM_STATE>
                        type: AnyPropertyType
                 long-offset: 00000000
 5.79:                                    32 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: GetProperty
                                                      format: 00
                                                        type: <NONE>
                                                 bytes-after: 00000000
 5.79: Client (pid 5555 spectacle) -->    8 bytes
         ............REQUEST: QueryTree
                      window: WIN 01c28920
 5.79:                                    32 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: QueryTree
                                                        root: WIN 00000148
                                                      parent: WIN 00000148
 5.79: Client (pid 5555 spectacle) -->    8 bytes
         ............REQUEST: GetGeometry
                    drawable: DWB 01c28920
 5.79:                                    32 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: GetGeometry
                                                       depth: 00
                                                        root: WIN 00000148
                                                           x: 0
                                                           y: 0
                                                       width: 1180
                                                      height: 05a0
                                                border-width: 0000
 5.79: Client (pid 5555 spectacle) -->    8 bytes
         ............REQUEST: GetGeometry
                    drawable: DWB 00000148
 5.79:                                    32 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: GetGeometry
                                                       depth: 18
                                                        root: WIN 00000148
                                                           x: 0
                                                           y: 0
                                                       width: 1180
                                                      height: 05a0
                                                border-width: 0000
 5.79: Client (pid 5555 spectacle) -->    8 bytes
         ............REQUEST: GetWindowAttributes
                      window: WIN 01c28920
 5.79:                                    44 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: GetWindowAttributes
                                               backing-store: NotUseful
                                                      visual: VIS 000000a9
                                                       class: InputOnly
                                                 bit-gravity: Forget
                                                 win-gravity: NorthWest
                                              backing-planes: ffffffff
                                               backing-pixel: 00000000
                                                  save-under: False
                                            map-is-installed: False
                                                   map-state: Viewable
                                           override-redirect: True
                                                    colormap: None
                                             all-event-masks: ButtonPress | ButtonRelease | PointerMotion
                                             your-event-mask: 0
                                        do-not-propagate-mask: 0
 5.79: Client (pid 5555 spectacle) -->   84 bytes
         ............REQUEST: CreatePixmap
                       depth: 00
                   pixmap-id: PXM 00a00019
                    drawable: DWB 01c28920
                       width: 1180
                      height: 05a0
         ............REQUEST: CreateGC
          graphic-context-id: GXC 00a0001a
                    drawable: DWB 00a00019
                  value-mask: subwindow-mode
                  value-list:
                      subwindow-mode: IncludeInferiors
         ............REQUEST: CopyArea
                src-drawable: DWB 01c28920
                dst-drawable: DWB 00a00019
                          gc: GXC 00a0001a
                       src-x: 0
                       src-y: 0
                       dst-x: 0
                       dst-y: 0
                       width: 1180
                      height: 05a0
         ............REQUEST: GetImage
                      format: ZPixmap
                    drawable: DWB 00a00019
                           x: 0
                           y: 0
                       width: 1180
                      height: 05a0
                  plane-mask: ffffffff
 5.79:                                   128 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............ERROR: Value
                                                   bad value: 0
                                                minor opcode: 0000
                                                major opcode: 35
                                         ..............ERROR: Drawable
                                             bad resource id: 00a00019
                                                minor opcode: 0000
                                                major opcode: 37
                                         ..............ERROR: Drawable
                                             bad resource id: 00a00019
                                                minor opcode: 0000
                                                major opcode: 3e
                                         ..............ERROR: Drawable
                                             bad resource id: 00a00019
                                                minor opcode: 0000
                                                major opcode: 49
 5.79: Client (pid 5555 spectacle) -->   24 bytes
         ............REQUEST: FreeGC
                          gc: GXC 00a0001a
         ............REQUEST: FreePixmap
                      pixmap: PXM 00a00019
         ............REQUEST: GetGeometry
                    drawable: DWB 00000148
 5.79:                                    64 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............ERROR: GContext
                                             bad resource id: 00a00019
                                                minor opcode: 0000
                                                major opcode: 3c
                                         ..............ERROR: Pixmap
                                             bad resource id: 00a00019
                                                minor opcode: 0000
                                                major opcode: 36
 5.79:                                    32 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: GetGeometry
                                                       depth: 18
                                                        root: WIN 00000148
                                                           x: 0
                                                           y: 0
                                                       width: 1180
                                                      height: 05a0
                                                border-width: 0000
 5.79: Client (pid 5555 spectacle) -->    8 bytes
         ............REQUEST: GetGeometry
                    drawable: DWB 00000148
 5.79:                                    32 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: GetGeometry
                                                       depth: 18
                                                        root: WIN 00000148
                                                           x: 0
                                                           y: 0
                                                       width: 1180
                                                      height: 05a0
                                                border-width: 0000
 5.79: Client (pid 5555 spectacle) -->   16 bytes
         ............REQUEST: TranslateCoordinates
                  src-window: WIN 00000148
                  dst-window: WIN 00000148
                       src-x: 0
                       src-y: 0
 5.79:                                    32 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: TranslateCoordinates
                                                 same-screen: True
                                                       child: WIN 01cd28df
                                                       dst-x: 0
                                                       dst-y: 0
 5.79: Client (pid 5555 spectacle) -->    8 bytes
         ............REQUEST: GetWindowAttributes
                      window: WIN 00000148
 5.79:                                    44 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............REPLY: GetWindowAttributes
                                               backing-store: NotUseful
                                                      visual: VIS 000000a9
                                                       class: InputOutput
                                                 bit-gravity: NorthWest
                                                 win-gravity: NorthWest
                                              backing-planes: ffffffff
                                               backing-pixel: 00000000
                                                  save-under: False
                                            map-is-installed: True
                                                   map-state: Viewable
                                           override-redirect: False
                                                    colormap: CMP 000000a8
                                             all-event-masks: KeyPress | KeyRelease | EnterWindow | LeaveWindow | KeymapState | Exposure | StructureNotify | SubstructureNotify | SubstructureRedirect | FocusChange | PropertyChange | ColormapChange
                                             your-event-mask: EnterWindow | LeaveWindow | StructureNotify | PropertyChange
                                        do-not-propagate-mask: 0
 5.79: Client (pid 5555 spectacle) -->   84 bytes
         ............REQUEST: CreatePixmap
                       depth: 18
                   pixmap-id: PXM 00a0001b
                    drawable: DWB 00000148
                       width: 1180
                      height: 05a0
         ............REQUEST: CreateGC
          graphic-context-id: GXC 00a0001c
                    drawable: DWB 00a0001b
                  value-mask: subwindow-mode
                  value-list:
                      subwindow-mode: IncludeInferiors
         ............REQUEST: CopyArea
                src-drawable: DWB 00000148
                dst-drawable: DWB 00a0001b
                          gc: GXC 00a0001c
                       src-x: 0
                       src-y: 0
                       dst-x: 0
                       dst-y: 0
                       width: 1180
                      height: 05a0
         ............REQUEST: GetImage
                      format: ZPixmap
                    drawable: DWB 00a0001b
                           x: 0
                           y: 0
                       width: 1180
                      height: 05a0
                  plane-mask: ffffffff
 5.79:                                    64 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............EVENT: NoExposure
                                                    drawable: DWB 00a0001b
                                                minor-opcode: 0000
                                                major-opcode: 3e
 5.80:                                  32768 bytes <-- X11 Server (pid 29653 Xorg)
 5.80:                                  32768 bytes <-- X11 Server (pid 29653 Xorg)
 5.80:                                  32768 bytes <-- X11 Server (pid 29653 Xorg)
...
 5.89:                                  32768 bytes <-- X11 Server (pid 29653 Xorg)
 5.89:                                  26560 bytes <-- X11 Server (pid 29653 Xorg)

                                         ..............REPLY: GetImage
                                                       depth: 18
                                                      visual: None
 7.35: Client (pid 5555 spectacle) -->   96 bytes
         ............REQUEST: FreeGC
                          gc: GXC 00a0001c
         ............REQUEST: FreePixmap
                      pixmap: PXM 00a0001b
         ............REQUEST: ChangeProperty
                        mode: Replace
                      window: WIN 00a0000e
                    property: <_NET_WM_NAME>
                        type: <UTF8_STRING>
                      format: 08
                        data: "Unsaved* \342\200\224 Spectacle"
         ............REQUEST: ChangeProperty
                        mode: Replace
                      window: WIN 00a0000e
                    property: <WM_NAME>
                        type: <STRING>
                      format: 08
                        data: "Unsaved*"
 7.36:                                    64 bytes <-- X11 Server (pid 29653 Xorg)
                                         ..............EVENT: PropertyNotify
                                                      window: WIN 00a0000e
                                                        atom: <_NET_WM_NAME>
                                                        time: TIM 0e37d6a0
                                                       state: NewValue
                                         ..............EVENT: PropertyNotify
                                                      window: WIN 00a0000e
                                                        atom: <WM_NAME>
                                                        time: TIM 0e37d6a0
                                                       state: NewValue