UI: Limit preview scrolling

This prevents the preview from being scrolled on forever.

UI/window-basic-main.cpp

@@ -4665,6 +4665,7 @@ void OBSBasic::ResizePreview(uint32_t cx, uint32_t cy)
 	obs_get_video_info(&ovi);
 
 	if (isFixedScaling) {
+		ui->preview->ClampScrollingOffsets();
 		previewScale = ui->preview->GetScalingAmount();
 		GetCenterPosFromFixedScale(
 			int(cx), int(cy),

UI/window-basic-preview.cpp

@@ -9,6 +9,7 @@
 #include "window-basic-main.hpp"
 #include "obs-app.hpp"
 #include "platform.hpp"
+#include "display-helpers.hpp"
 
 #define HANDLE_RADIUS 4.0f
 #define HANDLE_SEL_RADIUS (HANDLE_RADIUS * 1.5f)
@@ -2606,3 +2607,26 @@ void OBSBasicPreview::DrawSpacingHelpers()
 	vec3_set(&end, 1.0f, right.y, 1.0f);
 	RenderSpacingHelper(3, start, end, viewport, pixelRatio);
 }
+
+void OBSBasicPreview::ClampScrollingOffsets()
+{
+	obs_video_info ovi;
+	obs_get_video_info(&ovi);
+
+	QSize targetSize = GetPixelSize(this);
+
+	vec3 target, offset;
+	vec3_set(&target, (float)targetSize.width(), (float)targetSize.height(),
+		 1.0f);
+
+	vec3_set(&offset, (float)ovi.base_width, (float)ovi.base_height, 1.0f);
+	vec3_mulf(&offset, &offset, scalingAmount);
+
+	vec3_sub(&offset, &offset, &target);
+
+	vec3_mulf(&offset, &offset, 0.5f);
+	vec3_maxf(&offset, &offset, 0.0f);
+
+	scrollingOffset.x = std::clamp(scrollingOffset.x, -offset.x, offset.x);
+	scrollingOffset.y = std::clamp(scrollingOffset.y, -offset.y, offset.y);
+}

UI/window-basic-preview.hpp

@@ -197,4 +197,5 @@ public:
 	int spacerPx[4] = {0};
 
 	void DrawSpacingHelpers();
+	void ClampScrollingOffsets();
 };