code-editor: make Backspace remove full indents at the start of a line

static/code-editor.js

@@ -26,6 +26,8 @@ export class CodeEditor extends HTMLElement {
         this.textArea.rows = 1;
 
         this.keyShortcuts = {
+            Backspace: () => this.backspace(),
+
             Enter: () => this.enter(),
 
             Tab: () => this.tab(),
@@ -251,6 +253,28 @@ export class CodeEditor extends HTMLElement {
         }
     }
 
+    backspace() {
+        this.pushHistory({ allowMerge: true });
+
+        let selection = this.getSelection();
+        selection.collapse();
+
+        let start = getLineStart(this.code, selection.start);
+        let leading = this.code.substring(start, selection.cursor);
+        let isAtIndent = /^ +$/.test(leading);
+        let positionInLine = selection.cursor - start;
+        let charactersToRemove = isAtIndent
+            ? this.indentWidth - (positionInLine % this.indentWidth)
+            : 1;
+
+        selection.cursor -= charactersToRemove;
+        selection.clampCursor(this.code);
+        this.replace(selection, "");
+        selection.collapse();
+
+        this.setSelection(selection);
+    }
+
     enter() {
         this.pushHistory({ allowMerge: false });
 
@@ -335,16 +359,20 @@ export class Selection {
         this.cursor = Math.max(0, Math.min(this.cursor, text.length));
     }
 
+    collapse() {
+        this.anchor = this.cursor;
+    }
+
     set(text, n) {
         this.cursor = n;
         this.clampCursor(text);
-        this.anchor = this.cursor;
+        this.collapse();
     }
 
     advance(text, n) {
         this.cursor += n;
         this.clampCursor(text);
-        this.anchor = this.cursor;
+        this.collapse();
     }
 }