LibWeb: Partially implement <textarea>'s selectionStart and selectionEnd

This implementation pretends we never have a selection. GitHub relies on
these values to know where to insert text corresponding to file uploads.

Tests/LibWeb/Text/expected/textarea-selection-start-selection-end.txt

@@ -0,0 +1,4 @@
+Well hello friends   selectionStart: 0
+selectionEnd: 0
+selectionStart: 18
+selectionEnd: 18

Tests/LibWeb/Text/input/textarea-selection-start-selection-end.html

@@ -0,0 +1,15 @@
+<textarea id="textarea"></textarea>
+<script src="include.js"></script>
+<script type="text/javascript">
+    test(() => {
+        let textarea = document.getElementById("textarea");
+        textarea.focus();
+
+        println(`selectionStart: ${textarea.selectionStart}`);
+        println(`selectionEnd: ${textarea.selectionEnd}`);
+
+        textarea.value = "Well hello friends";
+        println(`selectionStart: ${textarea.selectionStart}`);
+        println(`selectionEnd: ${textarea.selectionEnd}`);
+    });
+</script>

Userland/Libraries/LibWeb/HTML/HTMLTextAreaElement.cpp

@@ -201,6 +201,63 @@ void HTMLTextAreaElement::set_custom_validity(String const& error)
     dbgln("(STUBBED) HTMLTextAreaElement::set_custom_validity(\"{}\"). Called on: {}", error, debug_description());
 }
 
+// https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#dom-textarea/input-selectionstart
+WebIDL::UnsignedLong HTMLTextAreaElement::selection_start() const
+{
+    // 1. If this element is an input element, and selectionStart does not apply to this element, return null.
+
+    // 2. If there is no selection, return the code unit offset within the relevant value to the character that
+    //    immediately follows the text entry cursor.
+    if (auto const* browsing_context = document().browsing_context()) {
+        if (auto cursor = browsing_context->cursor_position())
+            return cursor->offset();
+    }
+
+    // FIXME: 3. Return the code unit offset within the relevant value to the character that immediately follows the start of
+    //           the selection.
+    return 0;
+}
+
+// https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#textFieldSelection:dom-textarea/input-selectionstart-2
+WebIDL::ExceptionOr<void> HTMLTextAreaElement::set_selection_start(WebIDL::UnsignedLong)
+{
+    // 1. If this element is an input element, and selectionStart does not apply to this element, throw an
+    //    "InvalidStateError" DOMException.
+
+    // FIXME: 2. Let end be the value of this element's selectionEnd attribute.
+    // FIXME: 3. If end is less than the given value, set end to the given value.
+    // FIXME: 4. Set the selection range with the given value, end, and the value of this element's selectionDirection attribute.
+    return {};
+}
+
+// https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#dom-textarea/input-selectionend
+WebIDL::UnsignedLong HTMLTextAreaElement::selection_end() const
+{
+    // 1. If this element is an input element, and selectionEnd does not apply to this element, return null.
+
+    // 2. If there is no selection, return the code unit offset within the relevant value to the character that
+    //    immediately follows the text entry cursor.
+    if (auto const* browsing_context = document().browsing_context()) {
+        if (auto cursor = browsing_context->cursor_position())
+            return cursor->offset();
+    }
+
+    // FIXME: 3. Return the code unit offset within the relevant value to the character that immediately follows the end of
+    //           the selection.
+    return 0;
+}
+
+// https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#textFieldSelection:dom-textarea/input-selectionend-3
+WebIDL::ExceptionOr<void> HTMLTextAreaElement::set_selection_end(WebIDL::UnsignedLong)
+{
+    // 1. If this element is an input element, and selectionEnd does not apply to this element, throw an
+    //    "InvalidStateError" DOMException.
+
+    // FIXME: 2. Set the selection range with the value of this element's selectionStart attribute, the given value, and the
+    //           value of this element's selectionDirection attribute.
+    return {};
+}
+
 // https://html.spec.whatwg.org/multipage/form-elements.html#dom-textarea-maxlength
 WebIDL::Long HTMLTextAreaElement::max_length() const
 {

Userland/Libraries/LibWeb/HTML/HTMLTextAreaElement.h

@@ -84,6 +84,12 @@ public:
     bool report_validity();
     void set_custom_validity(String const& error);
 
+    WebIDL::UnsignedLong selection_start() const;
+    WebIDL::ExceptionOr<void> set_selection_start(WebIDL::UnsignedLong);
+
+    WebIDL::UnsignedLong selection_end() const;
+    WebIDL::ExceptionOr<void> set_selection_end(WebIDL::UnsignedLong);
+
     WebIDL::Long max_length() const;
     WebIDL::ExceptionOr<void> set_max_length(WebIDL::Long);
 

Userland/Libraries/LibWeb/HTML/HTMLTextAreaElement.idl

@@ -35,8 +35,8 @@ interface HTMLTextAreaElement : HTMLElement {
     // FIXME: readonly attribute NodeList labels;
 
     // FIXME: undefined select();
-    // FIXME: attribute unsigned long selectionStart;
-    // FIXME: attribute unsigned long selectionEnd;
+    attribute unsigned long selectionStart;
+    attribute unsigned long selectionEnd;
     // FIXME: attribute DOMString selectionDirection;
     // FIXME: undefined setRangeText(DOMString replacement);
     // FIXME: undefined setRangeText(DOMString replacement, unsigned long start, unsigned long end, optional SelectionMode selectionMode = "preserve");