This patches allows

1. when the selection is shadow-crossing, dragging it will have the
   correct the drag image correctly displayed. It's hard to test, so
   no tests for this.

2. The selection can now be serialized and dropped.
   test_drag_drop_shadow_crossing_selection.html is the test for
   this.

Backed out for causing mochitest failures on test_synthmousemove_after_dnd.html

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• Failure Log for TV
• Failure Log for mochitest

[task 2024-08-26T15:03:13.471Z] 15:03:13     INFO - TEST-START | layout/base/tests/test_synthmousemove_after_dnd.html
[task 2024-08-26T15:03:14.322Z] 15:03:14     INFO - TEST-INFO | started process screentopng
[task 2024-08-26T15:03:18.074Z] 15:03:18     INFO - TEST-INFO | screentopng: exit 0
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - Buffered messages logged at 15:03:14
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - must wait for load
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - Buffered messages finished
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - TEST-UNEXPECTED-FAIL | layout/base/tests/test_synthmousemove_after_dnd.html | uncaught exception - Error: event target of "mousedown" is not srcElement nor its descendant at onMouseDown@http://mochi.test:8888/tests/SimpleTest/EventUtils.js:3577:15
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - synthesizeMouseAtPoint@http://mochi.test:8888/tests/SimpleTest/EventUtils.js:788:32
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - synthesizeMouse@http://mochi.test:8888/tests/SimpleTest/EventUtils.js:553:10
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - synthesizePlainDragAndDrop@http://mochi.test:8888/tests/SimpleTest/EventUtils.js:3584:22
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - async*test_basic@http://mochi.test:8888/tests/layout/base/tests/test_synthmousemove_after_dnd.html:39:31
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - @http://mochi.test:8888/tests/layout/base/tests/test_synthmousemove_after_dnd.html:52:5
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - async*SimpleTest.waitForFocus/<@http://mochi.test:8888/tests/SimpleTest/SimpleTest.js:1058:13
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO - 
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO -     simpletestOnerror@SimpleTest/SimpleTest.js:2044:18
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO -     synthesizeMouseAtPoint@SimpleTest/EventUtils.js:788:32
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO -     synthesizeMouse@SimpleTest/EventUtils.js:553:10
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO -     synthesizePlainDragAndDrop@SimpleTest/EventUtils.js:3584:22
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO -     async*test_basic@layout/base/tests/test_synthmousemove_after_dnd.html:39:31
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO -     @layout/base/tests/test_synthmousemove_after_dnd.html:52:5
[task 2024-08-26T15:03:18.085Z] 15:03:18     INFO -     async*SimpleTest.waitForFocus/<@SimpleTest/SimpleTest.js:1058:13
[task 2024-08-26T15:03:18.086Z] 15:03:18     INFO - GECKO(5050) | JavaScript error: http://mochi.test:8888/tests/SimpleTest/EventUtils.js, line 3577: Error: event target of "mousedown" is not srcElement nor its descendant
[task 2024-08-26T15:03:18.091Z] 15:03:18     INFO - GECKO(5050) | MEMORY STAT | vsize 2651MB | residentFast 209MB | heapAllocated 26MB
[task 2024-08-26T15:03:18.095Z] 15:03:18     INFO - GECKO(5050) | JavaScript error: http://mochi.test:8888/tests/SimpleTest/EventUtils.js, line 3619: Error: event target of "dragstart" is not srcElement nor its descendant
[task 2024-08-26T15:03:18.100Z] 15:03:18     INFO - GECKO(5050) | JavaScript error: http://mochi.test:8888/tests/SimpleTest/EventUtils.js, line 3861: Error: event target of "dragend" is not srcElement nor its descendant
[task 2024-08-26T15:03:18.102Z] 15:03:18     INFO - GECKO(5050) | [Parent 5050, Main Thread] WARNING: NS_ENSURE_TRUE(dragSession) failed: file /builds/worker/checkouts/gecko/dom/ipc/BrowserParent.cpp:4003
[task 2024-08-26T15:03:18.104Z] 15:03:18     INFO - GECKO(5050) | [Parent 5050, Main Thread] WARNING: NS_ENSURE_TRUE(dragSession) failed: file /builds/worker/checkouts/gecko/dom/ipc/BrowserParent.cpp:4003
[task 2024-08-26T15:03:18.106Z] 15:03:18     INFO - TEST-PASS | layout/base/tests/test_synthmousemove_after_dnd.html | test_basic: The drop target should have hover state 
[task 2024-08-26T15:03:18.109Z] 15:03:18    ERROR - TEST-UNEXPECTED-FAIL | SimpleTest | this test already called finish!
[task 2024-08-26T15:03:18.121Z] 15:03:18     INFO - TEST-UNEXPECTED-ERROR | layout/base/tests/test_synthmousemove_after_dnd.html | called finish() multiple times
[task 2024-08-26T15:03:18.122Z] 15:03:18     INFO - TEST-INFO took 1231ms
[task 2024-08-26T15:03:18.124Z] 15:03:18     INFO - TEST-OK | layout/base/tests/test_synthmousemove_after_dnd.html
[task 2024-08-26T15:03:18.125Z] 15:03:18     INFO - TEST-START | layout/base/tests/test_transformed_scrolling_repaints.html

Backed out for causing mochitests failures in test_drag_drop_shadow_crossing_selection.html.

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• Failure Log
• Failure line: TEST-UNEXPECTED-FAIL | dom/base/test/test_drag_drop_shadow_crossing_selection.html | undefined assertion name - got "<span id="outer1">ter1</span>\r\n <div id="host">\r\n <span>Inner1</span>\r\n <span>Inner2</span>\r\n <span id="inner3">Inner</span></div>", expected "<span id="outer1">ter1</span>\n <div id="host">\n <span>Inner1</span>\n <span>Inner2</span>\n <span id="inner3">Inner</span></div>"

https://hg.mozilla.org/mozilla-central/rev/5c6272143175

We received a backout request for this bug
Backed out changeset 5c6272143175 (Bug 1900426) for causing Bug 1915027, Bug 1915620, Bug 1915816
Link: https://hg.mozilla.org/integration/autoland/rev/a6477f62a4142874a09841e7e793bddb77f1c7b9

Backout merged in central: https://hg.mozilla.org/mozilla-central/rev/a6477f62a4142874a09841e7e793bddb77f1c7b9

https://hg.mozilla.org/mozilla-central/rev/39f90c776ee1