CSS: Avoid unit-conversion interference from CSS upper bounds

Fixes gh-2144
Closes gh-3745

1 files changed, 19 insertions, 17 deletions

diff --git a/src/css/adjustCSS.js b/src/css/adjustCSS.js
index ded3a1666..626ec7475 100644
--- a/src/css/adjustCSS.js
+++ b/src/css/adjustCSS.js
@@ -6,8 +6,7 @@ define( [
 "use strict";
 
 function adjustCSS( elem, prop, valueParts, tween ) {
-	var adjusted,
-		scale = 1,
+	var adjusted, scale,
 		maxIterations = 20,
 		currentValue = tween ?
 			function() {
@@ -25,30 +24,33 @@ function adjustCSS( elem, prop, valueParts, tween ) {
 
 	if ( initialInUnit && initialInUnit[ 3 ] !== unit ) {
 
+		// Support: Firefox <=54
+		// Halve the iteration target value to prevent interference from CSS upper bounds (gh-2144)
+		initial = initial / 2;
+
 		// Trust units reported by jQuery.css
 		unit = unit || initialInUnit[ 3 ];
 
-		// Make sure we update the tween properties later on
-		valueParts = valueParts || [];
-
 		// Iteratively approximate from a nonzero starting point
 		initialInUnit = +initial || 1;
 
-		do {
-
-			// If previous iteration zeroed out, double until we get *something*.
-			// Use string for doubling so we don't accidentally see scale as unchanged below
-			scale = scale || ".5";
+		while ( maxIterations-- ) {
 
-			// Adjust and apply
-			initialInUnit = initialInUnit / scale;
+			// Evaluate and update our best guess (doubling guesses that zero out).
+			// Finish if the scale equals or crosses 1 (making the old*new product non-positive).
 			jQuery.style( elem, prop, initialInUnit + unit );
+			if ( ( 1 - scale ) * ( 1 - ( scale = currentValue() / initial || 0.5 ) ) <= 0 ) {
+				maxIterations = 0;
+			}
+			initialInUnit = initialInUnit / scale;
+
+		}
 
-		// Update scale, tolerating zero or NaN from tween.cur()
-		// Break the loop if scale is unchanged or perfect, or if we've just had enough.
-		} while (
-			scale !== ( scale = currentValue() / initial ) && scale !== 1 && --maxIterations
-		);
+		initialInUnit = initialInUnit * 2;
+		jQuery.style( elem, prop, initialInUnit + unit );
+
+		// Make sure we update the tween properties later on
+		valueParts = valueParts || [];
 	}
 
 	if ( valueParts ) {