Per the spec, selectionStart and selectionEnd should return the same
values regardless of the selectionDirection. (That is, selectionStart is
always less than or equal to selectionEnd; the direction then implies
which of selectionStart or selectionEnd is the cursor position.)
There was no explicit WPT test for this, so I added one.
This bug was initially quite hard to wrap my head around, and I think
part of the problem is the code in TextInput. Therefore, in the process
of fixing it I have refactored the implementation of TextInput:
* Rename selection_begin to selection_origin. This value doesn't
necessarily correspond directly to the selectionStart DOM value - in
the case of a backward selection, it corresponds to selectionEnd.
I feel that "origin" doesn't imply a specific ordering as strongly as
"begin" (or "start" for that matter) does.
* In various other cases where "begin" is used as a synonym for "start",
just use "start" for consistency.
* Implement selection_start() and selection_end() methods (and their
_offset() variants) which directly correspond to their DOM
equivalents.
* Rename other related methods to make them less wordy and more
consistent / intention-revealing.
* Add assertions to assert_ok_selection() to ensure that our assumptions
about the ordering of selection_origin and edit_point are met. This
then revealed a bug in adjust_selection_for_horizontal_change() where
the value of selection_direction was not maintained correctly (causing
a unit test failure when the new assertion failed).
The selection API only applies to certain <input> types:
https://html.spec.whatwg.org/multipage/#do-not-apply
This commit ensures that we handle that correctly.
Some notes:
1. TextControl::set_dom_selection_direction now calls
set_selection_range(), which means that setting selectionDirection will
now fire a selection event, as it should per the spec.
2. There is a test for the firing of the select event in
tests/wpt/web-platform-tests/html/semantics/forms/textfieldselection/select-event.html,
however the test did not run due to this syntax error:
(pid:26017) "ERROR:script::dom::bindings::error: Error at http://web-platform.test:8000/html/semantics/forms/textfieldselection/select-event.html:50:11 missing = in const declaration"
This happens due to the us of the "for (const foo of ...)" construct.
Per https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of
this should actually work, so it's somewhat unsatisfying to have to
change the test.
4. If an <input>'s type is unset, it defaults to a text, and the
selection API applies. Also, if an <input>'s type is set to an
invalid value, it defaults to a text too. I've expanded the tests
to account for this second case.
The API for text control selection is the same for both <input> and
<textarea>:
https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#textFieldSelection
Before this change, they had similar but not identical implementations
with duplicate code. Now there is a common TextControl trait which
contains the implementation used by both. As a result, some previously
failing tests now pass.
