[fix] [Visual Editor] [LESS] [content] create stacking context differently

b96ab7bd created a new CSS stacking context[0-1] for .content.
Unfortunately, this triggered a browser bug[2] which affects VE edit
cards (an unwanted margin appears around them). This also occurs when
creating a stacking context with filters. The issue does not occur when
the stacking context is created by changing position or opacity. Replace
the identity transform a `position: relative`.

[0] https://philipwalton.com/articles/what-no-one-told-you-about-z-index/
[1] https://stackoverflow.com/questions/16148007/which-css-properties-create-a-stacking-context?answertab=votes#tab-top
[2] https://bugs.chromium.org/p/chromium/issues/detail?id=20574

Bug: T230154
Change-Id: I23ad776f0bcde8f7eec5ffbdefb8f0825db084ec

resources/skins.minerva.base.styles/common.less

@@ -47,7 +47,9 @@
 }
 
 .content {
-	// Create a new stacking context.
-	transform: scaleY( 1 );
+	// Create a new stacking context. `transform: translateZ( 0 );` would be preferred but that
+	// triggers a browser bug which affects the edit cards in Visual Editor:
+	// https://bugs.chromium.org/p/chromium/issues/detail?id=20574
+	position: relative;
 	z-index: @z-indexBase;
 }