CSS: Avoid unit-conversion interference from CSS upper bounds

Fixes gh-2144
Closes gh-3745

3 files changed, 37 insertions, 18 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 ) {
diff --git a/test/unit/css.js b/test/unit/css.js
index 4533e4692..79cce0549 100644
--- a/test/unit/css.js
+++ b/test/unit/css.js
@@ -271,6 +271,22 @@ QUnit.test( "css() non-px relative values (gh-1711)", function( assert ) {
 	add( "lineHeight",  50,  "%" );
 } );
 
+QUnit.test( "css() mismatched relative values with bounded styles (gh-2144)", function( assert ) {
+	assert.expect( 1 );
+
+	var right,
+		$container = jQuery( "<div/>" )
+			.css( { position: "absolute", width: "400px", fontSize: "4px" } )
+			.appendTo( "#qunit-fixture" ),
+		$el = jQuery( "<div/>" )
+			.css( { position: "absolute", left: "50%", right: "50%" } )
+			.appendTo( $container );
+
+	$el.css( "right", "-=25em" );
+	assert.equal( Math.round( parseFloat( $el.css( "right" ) ) ), 100,
+		"Constraints do not interfere with unit conversion" );
+} );
+
 QUnit.test( "css(String, Object)", function( assert ) {
 	assert.expect( 19 );
 	var j, div, display, ret, success;
diff --git a/test/unit/effects.js b/test/unit/effects.js
index 54c7f7995..ec1669f54 100644
--- a/test/unit/effects.js
+++ b/test/unit/effects.js
@@ -1807,7 +1807,8 @@ QUnit.test( "animate does not change start value for non-px animation (#7109)",
 		}
 	} ).queue( function( next ) {
 		var ratio = computed[ 0 ] / actual;
-		assert.ok( ratio > 0.9 && ratio < 1.1, "Starting width was close enough" );
+		assert.ok( ratio > 0.9 && ratio < 1.1,
+			"Starting width was close enough (" + computed[ 0 ] + " approximates " + actual + ")" );
 		next();
 		parent.remove();
 	} );