FEATURE: do not insert smart list if SHIFT+Enter is pressed

A common pattern in the industry for bypassing smart lists is detection of
the shift key.

This information is not available in the "beforeinput" event but it always
fires afer keydown, so we track if shift is pressed on keydown.

app/assets/javascripts/discourse/app/components/d-editor.js

@@ -379,6 +379,10 @@ export default class DEditor extends Component {
           "beforeinput",
           this.onBeforeInputSmartList
         );
+        this._textarea.addEventListener(
+          "keydown",
+          this.onBeforeInputSmartListShiftDetect
+        );
         this._textarea.addEventListener("input", this.onInputSmartList);
       }
 
@@ -415,12 +419,19 @@ export default class DEditor extends Component {
     }
   }
 
+  @bind
+  onBeforeInputSmartListShiftDetect(event) {
+    this._shiftPressed = event.shiftKey;
+  }
+
   @bind
   onBeforeInputSmartList(event) {
     // This inputType is much more consistently fired in `beforeinput`
     // rather than `input`.
+    if (!this._shiftPressed) {
       this.handleSmartListAutocomplete = event.inputType === "insertLineBreak";
     }
+  }
 
   @bind
   onInputSmartList() {
@@ -490,6 +501,10 @@ export default class DEditor extends Component {
           "beforeinput",
           this.onBeforeInputSmartList
         );
+        this._textarea.removeEventListener(
+          "keydown",
+          this.onBeforeInputSmartListShiftDetect
+        );
         this._textarea.removeEventListener("input", this.onInputSmartList);
       }
     }