Fix assertion failure after panning

Don't assert that mPotentiallySelecting is true, as it's OK to
simply return early; the canvas' internal state should be fine.

Fixes #50.

app/imagecanvas.cpp

@@ -1224,7 +1224,13 @@ void ImageCanvas::updateOrMoveSelectionArea()
 
 void ImageCanvas::updateSelectionArea()
 {
-    Q_ASSERT(mPotentiallySelecting);
+    if (!mPotentiallySelecting) {
+        // updateSelectionArea() can be called by updateOrMoveSelectionArea() as a result
+        // of moving after panning (all without releasing the mouse). In that case,
+        // we can't be selecting, as we were just panning, so we return early.
+        // Previously we would assert that mPotentiallySelecting was true, but that's too strict.
+        return;
+    }
 
     QRect newSelectionArea(mPressScenePosition.x(), mPressScenePosition.y(),
         mCursorSceneX - mPressScenePosition.x(), mCursorSceneY - mPressScenePosition.y());

tests/tst_app.cpp

@@ -104,6 +104,7 @@ private Q_SLOTS:
     void flipPastedImage();
     void selectionEdgePan_data();
     void selectionEdgePan();
+    void panThenMoveSelection();
 
     void fillImageCanvas_data();
     void fillImageCanvas();
@@ -2673,6 +2674,44 @@ void tst_App::selectionEdgePan()
     QTest::mouseRelease(window, Qt::LeftButton, Qt::NoModifier, cursorWindowPos);
 }
 
+// https://github.com/mitchcurtis/slate/issues/50
+// When panning while having an active selection, and then releasing the space bar
+// and hence starting a drag, there would be an assertion failure (Q_ASSERT(mPotentiallySelecting))
+// in ImageCanvas::updateSelectionArea().
+void tst_App::panThenMoveSelection()
+{
+    QVERIFY2(createNewLayeredImageProject(), failureMessage);
+
+    QVERIFY2(switchTool(ImageCanvas::SelectionTool), failureMessage);
+
+    setCursorPosInScenePixels(100, 100);
+    QTest::mousePress(window, Qt::LeftButton, Qt::NoModifier, cursorWindowPos);
+
+    // Add an extra move otherwise the pan isn't started for some reason.
+    QTest::mouseMove(window, cursorWindowPos + QPoint(1, 1));
+
+    setCursorPosInScenePixels(120, 120);
+    QTest::mouseMove(window, cursorWindowPos);
+    QTest::mouseRelease(window, Qt::LeftButton, Qt::NoModifier, cursorWindowPos);
+    QVERIFY(layeredImageCanvas->hasSelection());
+
+    // Start panning.
+    QTest::keyPress(window, Qt::Key_Space);
+    QCOMPARE(window->cursor().shape(), Qt::OpenHandCursor);
+
+    QTest::mousePress(window, Qt::LeftButton, Qt::NoModifier, cursorWindowPos);
+    QCOMPARE(window->cursor().shape(), Qt::ClosedHandCursor);
+
+    // Stop panning but don't release the mouse.
+    QTest::keyRelease(window, Qt::Key_Space);
+
+    // Move the mouse slightly and it would previously crash with an assertion failure.
+    QTest::mouseMove(window, cursorWindowPos + QPoint(1, 1));
+
+    // Release to finish up.
+    QTest::mouseRelease(window, Qt::LeftButton, Qt::NoModifier, cursorWindowPos);
+}
+
 void tst_App::fillImageCanvas_data()
 {
     addImageProjectTypes();