add askIfShouldIgnoreMousedown()

src/mathquill.d.ts

@@ -137,6 +137,10 @@ declare namespace MathQuill {
       disableAutoSubstitutionInSubscripts?: boolean | { except: string };
       interpretTildeAsSim?: boolean;
       handlers?: HandlerOptions<BaseMathQuill<$>>;
+      askIfShouldIgnoreMousemove?: (
+        evt: MouseEvent,
+        rootElt: HTMLElement
+      ) => boolean;
     }
 
     interface Handler<MQClass> {

src/publicapi.ts

@@ -89,6 +89,11 @@ class Options {
   constructor(public version: 1 | 2 | 3) {}
 
   ignoreNextMousedown: (_el: MouseEvent) => boolean;
+  askIfShouldIgnoreMousemove: (
+    evt: MouseEvent,
+    rootDOM: HTMLElement
+  ) => boolean;
+
   substituteTextarea: () => HTMLElement;
   /** Only used in interface versions 1 and 2. */
   substituteKeyboardEvents: SubstituteKeyboardEvents;

src/services/mouse.ts

@@ -6,6 +6,11 @@ const ignoreNextMouseDownNoop = (_el: MouseEvent) => {
 };
 Options.prototype.ignoreNextMousedown = ignoreNextMouseDownNoop;
 
+const askIfShouldIgnoreMousemoveNoop = (_evt: MouseEvent, _el: HTMLElement) => {
+  return false;
+};
+Options.prototype.askIfShouldIgnoreMousemove = askIfShouldIgnoreMousemoveNoop;
+
 // Whenever edits to the tree occur, in-progress selection events
 // must be invalidated and selection changes must not be applied to
 // the edited tree. cancelSelectionOnEdit takes care of this.
@@ -62,11 +67,25 @@ class Controller_mouse extends Controller_latex {
     }
 
     var lastMousemoveTarget: HTMLElement | null = null;
-    function mousemove(e: Event) {
+    function mousemove(e: MouseEvent) {
+      if (
+        rootElement &&
+        cursor.options.askIfShouldIgnoreMousemove(e, rootElement)
+      )
+        return;
       lastMousemoveTarget = e.target as HTMLElement | null;
     }
     function onDocumentMouseMove(e: MouseEvent) {
-      if (!cursor.anticursor) cursor.startSelection();
+      if (
+        rootElement &&
+        cursor.options.askIfShouldIgnoreMousemove(e, rootElement)
+      )
+        return;
+
+      if (!cursor.anticursor) {
+        ctrlr.restoreLatexSelection(originalSelection);
+        cursor.startSelection();
+      }
       ctrlr.seek(lastMousemoveTarget, e.clientX, e.clientY).cursor.select();
       if (cursor.selection)
         cursor.controller.aria
@@ -131,6 +150,8 @@ class Controller_mouse extends Controller_latex {
       .seek(e.target as HTMLElement | null, e.clientX, e.clientY)
       .cursor.startSelection();
 
+    const originalSelection = ctrlr.exportLatexSelection().selection;
+
     rootElement?.addEventListener('mousemove', mousemove);
     ownerDocument?.addEventListener('mousemove', onDocumentMouseMove);
     ownerDocument?.addEventListener('mouseup', onDocumentMouseUp);

test/unit/mouse.test.js

@@ -69,6 +69,60 @@ suite('mouse', function () {
       endIndex: 12
     });
   });
+
+  test('askIfShouldIgnoreMousemove prevents drag selection', function () {
+    const mq = MQ.MathField($('<span></span>').appendTo('#mock')[0]);
+
+    let shouldIgnore = false;
+    mq.__controller.cursor.options.askIfShouldIgnoreMousemove = () => {
+      return shouldIgnore;
+    };
+
+    mq.latex('1+3+5+7+1');
+    const rect = mq.el().getBoundingClientRect();
+    const y = rect.top + 10;
+
+    const three = $('#mock .mq-digit:contains(3)');
+    const five = $('#mock .mq-digit:contains(5)');
+    const beforeOne = rect.left + 1;
+    const afterThree = three.get(0).getBoundingClientRect().right;
+    const afterFive = five.get(0).getBoundingClientRect().right;
+
+    // Baseline: normal drag selects
+    dispatchMouseEventAtPoint('mousedown', beforeOne, y);
+    dispatchMouseEventAtPoint('mousemove', afterThree, y);
+    assertDeepEqual(mq.selection(), {
+      latex: '1+3+5+7+1',
+      startIndex: 0,
+      endIndex: 3
+    });
+
+    // With ignore=true: drag should not extend selection
+    shouldIgnore = true;
+    dispatchMouseEventAtPoint('mousemove', afterFive, y);
+    assertDeepEqual(mq.selection(), {
+      latex: '1+3+5+7+1',
+      startIndex: 0,
+      endIndex: 3
+    });
+
+    // With ignore=false again: drag works
+    shouldIgnore = false;
+    dispatchMouseEventAtPoint('mousemove', afterFive + 1, y);
+    assertDeepEqual(mq.selection(), {
+      latex: '1+3+5+7+1',
+      startIndex: 0,
+      endIndex: 5
+    });
+
+    // can go back to 3 with ignore=false
+    dispatchMouseEventAtPoint('mousemove', afterThree, y);
+    assertDeepEqual(mq.selection(), {
+      latex: '1+3+5+7+1',
+      startIndex: 0,
+      endIndex: 3
+    });
+  });
 });
 
 function assertDeepEqual(a, b) {