part 3: Set an explicit whole-pixel-valued height on the textfield in…

… text-input-vertical-overflow-no-scroll.html.

This avoids triggering bug 1851066, per the comment in the test.  Bug 1851066
is a real bug, but it's quite subtle and is unrelated to what this WPT is
actually trying to test.

Differential Revision: https://phabricator.services.mozilla.com/D187241

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1802466
gecko-commit: fecbf97cc8db1ad76076fa902abbdc791fec8bd1
gecko-reviewers: ntim

css/css-writing-modes/forms/text-input-vertical-overflow-no-scroll.html

@@ -12,7 +12,22 @@
     .spacer {
         height: 100vh;
     }
-    input { font-family: monospace; }
+    input {
+        font-family: monospace;
+
+        /* Use an explicit whole-number-of-pixels height, to avoid tripping
+           Mozilla bug 1851066, regarding fractional scroll positions
+           triggering an async 1px adjustment to scrollTop. We can remove this
+           once that bug is fixed.
+           (Without this hack, the input element may end up with a height
+           that's a fractional number of pixels, depending on the font.
+           And then this test's call to 'scrollIntoView' puts us at a
+           fractional scroll-position, to bottom-align the input element. And
+           that's sufficient to trigger bug 1851066 and get an asynchronous 1px
+           adjustment to scrollTop, which this test doesn't expect.
+        */
+        height: 200px;
+    }
 </style>
 
 <div class="spacer"></div>