A good place to write a test for this would be a DumpAccessibilityTree test - see these docs: https://chromium.googlesource.com/chromium/src/+/lkgr/content/test/data/accessibility/readme.md - and follow the examples in content/test/data/accessibility/html/*coordinates*html for some specific tests involving coordinates and bounding boxes.

The fix may require two things:

* AXObject::GetRelativeBounds in third_party/WebKit/Source/modules/accessibility/AXObject.cpp may be returning the size of the whole page contents, instead of just the visible portion of the page, when IsWebArea() is true.

* Once AXObject::GetRelativeBounds correctly contains the bounds of the web page, AXTree::RelativeToTreeBounds should be able to do the clipping. Fix ui/accessibility/ax_tree.cc and add a new test to ui/accessibility/ax_tree_unittest.cc, then make the DumpAccessibilityTree test pass too.

My preferred behavior would be that the width and/or height are reduced to 1 pixel, not 0 pixels, because 0 pixels indicates an uninitialized or invalid bounding box.

So for example, if the window is 800 x 600 pixels, and the accessible object has coordinates (400, 700) size (50 x 50), the bounding box should be clipped to (400, 599) size (50 x 1).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/77cdaac7966c31125e514b7030bf699379c844e9

commit 77cdaac7966c31125e514b7030bf699379c844e9
Author: Katie Dektar <katydek@google.com>
Date: Tue Sep 19 21:28:30 2017

Clips accessible bounding box to nearest window.

For example, if a bounding box extends partially below of a window,
it will be shrunk in Y to match the border of the window. If a
bounding box is fully below a window, it will be decreased to a
height of 1 and placed at the very bottom of that window.

R=dmazzoni@chromium.org

Bug:  764065 
Change-Id: Ibb105874710ebb2a0f3a7623d81b10a3f51f09ca
Reviewed-on: https://chromium-review.googlesource.com/669949
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Katie Dektar <katydek@google.com>
Cr-Commit-Position: refs/heads/master@{#502954}
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/chrome/test/data/extensions/api_test/automation/tests/location_scaled/location_scaled.js
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/chrome/test/data/extensions/api_test/automation/tests/tabs/location.js
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/browser/accessibility/accessibility_win_browsertest.cc
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/browser/accessibility/browser_accessibility_manager_unittest.cc
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/browser/accessibility/dump_accessibility_tree_browsertest.cc
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-coordinates-cross-process-expected-blink.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-coordinates-cross-process-expected-mac.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-coordinates-cross-process-expected-win.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-coordinates-expected-blink.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-coordinates-expected-mac.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-coordinates-expected-win.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform-cross-process-expected-blink.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform-cross-process.html
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform-expected-blink.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform-expected-win.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform-nested-cross-process-expected-blink.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform-nested-cross-process.html
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform-nested-expected-blink.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform-nested.html
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform-scrolled-expected-blink.txt
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform-scrolled.html
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/iframe-transform.html
[add] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/window-crops-items-expected-blink.txt
[add] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/content/test/data/accessibility/html/window-crops-items.html
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/third_party/WebKit/Source/modules/accessibility/AXObject.cpp
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/ui/accessibility/ax_tree.cc
[modify] https://crrev.com/77cdaac7966c31125e514b7030bf699379c844e9/ui/accessibility/ax_tree_unittest.cc