LibWeb: Add new whitespace-preserving editing command

Major browsers seem to preserve `white-space: pre/pre-wrap` styles in a
`<div>` when deleting the current selection through an editing command.
The idiomatic way to support this is to have a command with a "relevant
CSS property" to make sure the value is recorded and restored where
appropriate, however, no such command exists.

Create a custom command (internal to Ladybird) that implements this
behavior.

Libraries/LibWeb/Editing/CommandNames.h

@@ -40,6 +40,7 @@ namespace Web::Editing::CommandNames {
     __ENUMERATE_COMMAND_NAME(justifyRight, "justifyRight")                           \
     __ENUMERATE_COMMAND_NAME(outdent, "outdent")                                     \
     __ENUMERATE_COMMAND_NAME(paste, "paste")                                         \
+    __ENUMERATE_COMMAND_NAME(preserveWhitespace, "preserveWhitespace")               \
     __ENUMERATE_COMMAND_NAME(redo, "redo")                                           \
     __ENUMERATE_COMMAND_NAME(removeFormat, "removeFormat")                           \
     __ENUMERATE_COMMAND_NAME(selectAll, "selectAll")                                 \

Libraries/LibWeb/Editing/Commands.cpp

@@ -2657,6 +2657,15 @@ static Array const commands {
         .preserves_overrides = true,
         .mapped_value = "formatOutdent"_fly_string,
     },
+    // AD-HOC: This is a Ladybird-specific formatting command that is not part of the spec. It has no action and as
+    //         such, it's not supported in userland (yet). The relevant CSS property `white-space` is used to indicate
+    //         that if this style value is found during editing commands, it is recorded and restored where necessary.
+    //         This is used to keep things like <div style="white-space: pre">..</div> intact when a selection is
+    //         deleted, for example.
+    CommandDefinition {
+        .command = CommandNames::preserveWhitespace,
+        .relevant_css_property = CSS::PropertyID::WhiteSpace,
+    },
     // https://w3c.github.io/editing/docs/execCommand/#the-removeformat-command
     CommandDefinition {
         .command = CommandNames::removeFormat,

Libraries/LibWeb/Editing/ExecCommand.cpp

@@ -360,10 +360,11 @@ WebIDL::ExceptionOr<bool> Document::query_command_supported(FlyString const& com
     if (!is_html_document())
         return WebIDL::InvalidStateError::create(realm(), "queryCommandSupported is only supported on HTML documents"_string);
 
-    // When the queryCommandSupported(command) method on the Document interface is invoked, the
-    // user agent must return true if command is supported and available within the current script
-    // on the current site, and false otherwise.
-    return Editing::find_command_definition(command).has_value();
+    // When the queryCommandSupported(command) method on the Document interface is invoked, the user agent must return
+    // true if command is supported and available within the current script on the current site, and false otherwise.
+    // AD-HOC: Supported commands should have an action defined. Currently, ::preserveWhitespace does not have one.
+    auto command_definition = Editing::find_command_definition(command);
+    return command_definition.has_value() && command_definition->action;
 }
 
 // https://w3c.github.io/editing/docs/execCommand/#querycommandvalue()

Libraries/LibWeb/Editing/Internal/Algorithms.cpp

@@ -3333,9 +3333,10 @@ Vector<RecordedNodeValue> record_the_values_of_nodes(Vector<GC::Ref<DOM::Node>>
     // 2. For each node in node list, for each command in the list "subscript", "bold", "fontName",
     //    "fontSize", "foreColor", "hiliteColor", "italic", "strikethrough", and "underline" in that
     //    order:
+    // AD-HOC: We include "preserveWhitespace" as well.
     Array const commands = { CommandNames::subscript, CommandNames::bold, CommandNames::fontName,
         CommandNames::fontSize, CommandNames::foreColor, CommandNames::hiliteColor, CommandNames::italic,
-        CommandNames::strikethrough, CommandNames::underline };
+        CommandNames::strikethrough, CommandNames::underline, CommandNames::preserveWhitespace };
     for (auto node : node_list) {
         for (auto command : commands) {
             // 1. Let ancestor equal node.

Tests/LibWeb/Text/expected/Editing/execCommand-preserveWhitespace.txt

@@ -0,0 +1,2 @@
+Before: foo<div style="white-space: pre">bar</div>
+After: foo<span style="white-space: pre;">bar</span>

Tests/LibWeb/Text/input/Editing/execCommand-preserveWhitespace.html

@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<script src="../include.js"></script>
+<div contenteditable>foo<div style="white-space: pre">bar</div></div>
+<script>
+test(() => {
+    var divElm = document.querySelector('div');
+    println(`Before: ${divElm.innerHTML}`);
+
+    // Put cursor before 'bar'
+    var range = document.createRange();
+    range.setStart(divElm.childNodes[1].childNodes[0], 0);
+    getSelection().addRange(range);
+
+    // Press backspace
+    document.execCommand('delete');
+
+    println(`After: ${divElm.innerHTML}`);
+});
+</script>