prefer transform scaling over zoom on hdpi displays

js/reveal.js

@@ -2243,10 +2243,12 @@
 					transformSlides( { layout: '' } );
 				}
 				else {
-					// Prefer zoom for scaling up so that content remains crisp.
+					// Zoom Scaling
-					// Don't use zoom to scale down since that can lead to shifts
+					// Content remains crisp no matter how much we scale. Side
-					// in text layout/line breaks.
+					// effects are minor differences in text layout and iframe
-					if( scale > 1 && features.zoom ) {
+					// viewports changing size. A 200x200 iframe viewport in a
+					// 2x zoomed presentation ends up having a 400x400 viewport.
+					if( scale > 1 && features.zoom && window.devicePixelRatio < 2 ) {
 						dom.slides.style.zoom = scale;
 						dom.slides.style.left = '';
 						dom.slides.style.top = '';
@@ -2254,7 +2256,10 @@
 						dom.slides.style.right = '';
 						transformSlides( { layout: '' } );
 					}
-					// Apply scale transform as a fallback
+					// Transform Scaling
+					// Content layout remains the exact same when scaled up.
+					// Side effect is content becoming blurred, especially with
+					// high scale values on ldpi screens.
 					else {
 						dom.slides.style.zoom = '';
 						dom.slides.style.left = '50%';