main: Preserve image view position as much as possible

This makes it so the current scroll percentage on both axes is saved
by most calls of refreshPreviews() prior to calling
resetPreviewLayout().

Fixes issue #14.

Author: irydacea

NEWS.md

@@ -11,6 +11,7 @@ Version 0.5.1
 
 ### Bug fixes
 
+* The image view no longer scrolls back to the center of the image when zooming in/out, selecting a different color range/palette/transform type, or closing Preferences (#14).
 * Fixed QuaZip-related build errors with Qt 6.7.2 and later when using `ENABLE_BUILTIN_IMAGE_PLUGINS`.
 
 ### Other changes

src/mainwindow.cpp

@@ -949,7 +949,7 @@ void MainWindow::dropEvent(QDropEvent* event)
 		updateWindowTitle(true, {}, ImageOriginDrop);
 	}
 
-	refreshPreviews();
+	refreshPreviews(false, false);
 	enableWorkArea(true);
 }
 
@@ -1072,7 +1072,7 @@ void MainWindow::openFile(const QString& fileName)
 	MosCurrentConfig().addRecentFile(imagePath_, originalImage_);
 	updateRecentFilesMenu();
 	updateWindowTitle(true, imagePath_);
-	refreshPreviews();
+	refreshPreviews(false, false);
 
 	enableWorkArea(true);
 }
@@ -1091,11 +1091,28 @@ void MainWindow::doReloadFile()
 	refreshPreviews();
 }
 
-void MainWindow::refreshPreviews(bool skipRerender)
+void MainWindow::refreshPreviews(bool skipRerender, bool keepPos)
 {
 	if (!hasImage() || signalsBlocked())
 		return;
 
+	QPointF previewPos{50.0, 50.0};
+
+	if (keepPos) {
+		switch (viewMode_)
+		{
+			case MosConfig::ImageViewSwipe:
+			case MosConfig::ImageViewOnionSkin:
+				previewPos = currentScrollPercent(ui->previewCompositeContainer);
+				break;
+			default:
+				// In reality we only need one container's position, as the other
+				// one will be automatically adjusted in tandem with whichever we
+				// choose to modify next
+				previewPos = currentScrollPercent(ui->previewOriginalContainer);
+		}
+	}
+
 	if (!skipRerender) {
 		switch (rcMode_)
 		{
@@ -1134,7 +1151,7 @@ void MainWindow::refreshPreviews(bool skipRerender)
 		case MosConfig::ImageViewSwipe:
 		case MosConfig::ImageViewOnionSkin:
 			ui->previewComposite->setImages(originalImage_, transformedImage_);
-			resetPreviewLayout(ui->previewCompositeContainer, ui->previewComposite);
+			resetPreviewLayout(ui->previewCompositeContainer, ui->previewComposite, previewPos);
 
 			ui->previewOriginal->clear();
 			ui->previewRc->clear();
@@ -1145,13 +1162,37 @@ void MainWindow::refreshPreviews(bool skipRerender)
 
 			ui->previewOriginal->setImage(originalImage_);
 			ui->previewRc->setImage(transformedImage_);
-			resetPreviewLayout(ui->previewOriginalContainer, ui->previewOriginal);
-			resetPreviewLayout(ui->previewRcContainer, ui->previewRc);
+			resetPreviewLayout(ui->previewOriginalContainer, ui->previewOriginal, previewPos);
+			resetPreviewLayout(ui->previewRcContainer, ui->previewRc, previewPos);
 	}
 }
 
+QPointF MainWindow::currentScrollPercent(QAbstractScrollArea* scrollArea) const
+{
+	QPointF res{50.0, 50.0};
+
+	if (!scrollArea)
+		return res;
+
+	auto* hScroll = scrollArea->horizontalScrollBar();
+	auto* vScroll = scrollArea->verticalScrollBar();
+
+	// If the scrollbars are locked because the image hasn't been zoomed in
+	// enough to allow scrolling, pretend that they are set to the halfway
+	// point on the relevant axis. This prevents zooming in from scrolling the
+	// view to the top left corner of the image every time.
+
+	if (hScroll && hScroll->minimum() != hScroll->maximum())
+		res.setX(100.0 * hScroll->value() / hScroll->maximum());
+	if (vScroll && vScroll->minimum() != vScroll->maximum())
+		res.setY(100.0 * vScroll->value() / vScroll->maximum());
+
+	return res;
+}
+
 void MainWindow::resetPreviewLayout(QAbstractScrollArea* scrollArea,
-									QWidget* previewWidget)
+									QWidget* previewWidget,
+									QPointF scrollPercent)
 {
 	if (!scrollArea || !previewWidget)
 		return;
@@ -1164,8 +1205,11 @@ void MainWindow::resetPreviewLayout(QAbstractScrollArea* scrollArea,
 	auto* hScroll = scrollArea->horizontalScrollBar();
 	auto* vScroll = scrollArea->verticalScrollBar();
 
-	hScroll->setValue(hScroll->maximum() / 2);
-	vScroll->setValue(vScroll->maximum() / 2);
+	auto hPerc = qBound(0.0, scrollPercent.x(), 100.0);
+	auto vPerc = qBound(0.0, scrollPercent.y(), 100.0);
+
+	hScroll->setValue(hScroll->maximum() * hPerc / 100.0);
+	vScroll->setValue(vScroll->maximum() * vPerc / 100.0);
 }
 
 void MainWindow::doSaveFile()
@@ -1281,7 +1325,7 @@ void MainWindow::setViewMode(MainWindow::ViewMode newViewMode)
 	ui->cbxViewMode->setCurrentIndex(viewMode_);
 
 	// Update preview widgets if applicable
-	refreshPreviews(true);
+	refreshPreviews(true, false);
 }
 
 void MainWindow::setRcMode(MainWindow::RcMode newRcMode)
@@ -1923,7 +1967,7 @@ void MainWindow::on_actionPaste_triggered()
 	imagePath_ = tr("Clipboard image") % ".png";
 	updateWindowTitle(true, {}, ImageOriginClipboard);
 
-	refreshPreviews();
+	refreshPreviews(false, false);
 	enableWorkArea(true);
 }
 

src/mainwindow.hpp

@@ -191,7 +191,7 @@ class MainWindow : public QMainWindow {
 	QStringList doSaveColorBlend(const QString& dirPath);
 	QStringList doSaveColorShift(const QString& dirPath);
 
-	void refreshPreviews(bool skipRerender = false);
+	void refreshPreviews(bool skipRerender = false, bool keepPos = true);
 
 	QString currentPaletteName(bool paletteSwitchMode = false) const;
 	ColorList currentPalette(bool paletteSwitchMode = false) const;
@@ -200,8 +200,11 @@ class MainWindow : public QMainWindow {
 
 	void setPreviewBackgroundColor(const QString& colorName);
 
+	QPointF currentScrollPercent(QAbstractScrollArea* scrollArea) const;
+
 	void resetPreviewLayout(QAbstractScrollArea* scrollArea,
-							QWidget* previewWidget);
+							QWidget* previewWidget,
+							QPointF scrollPercent);
 
 	void doCustomPreviewBgSelect();
 	void updateCustomPreviewBgIcon();