layout: Don't take collapsible top margin into account when determining

the float ceiling.

As the float ceiling is relative to the border box, not the margin box,
of the parent flow, top margin must not be included.

This exposed a pre-existing bug whereby margins are discarded if a block
contains only floats and no content, affecting the tests
`float_intrinsic_height.html` and `margins_inside_floats_a.html`. As a
workaround, some invisible content has been added to the bodies of both
tests.

components/layout/block.rs

@@ -1990,7 +1990,8 @@ pub struct ISizeConstraintSolution {
 }
 
 impl ISizeConstraintSolution {
-    pub fn new(inline_size: Au, margin_inline_start: Au, margin_inline_end: Au) -> ISizeConstraintSolution {
+    pub fn new(inline_size: Au, margin_inline_start: Au, margin_inline_end: Au)
+               -> ISizeConstraintSolution {
         ISizeConstraintSolution {
             inline_start: Au(0),
             inline_end: Au(0),

components/layout/model.rs

@@ -174,7 +174,12 @@ impl MarginCollapseInfo {
 
     pub fn current_float_ceiling(&mut self) -> Au {
         match self.state {
-            MarginCollapseState::AccumulatingCollapsibleTopMargin => self.block_start_margin.collapse(),
+            MarginCollapseState::AccumulatingCollapsibleTopMargin => {
+                // We do not include the top margin in the float ceiling, because the float flow
+                // needs to be positioned relative to our *border box*, not our margin box. See
+                // `tests/ref/float_under_top_margin_a.html`.
+                Au(0)
+            }
             MarginCollapseState::AccumulatingMarginIn => self.margin_in.collapse(),
         }
     }
@@ -182,18 +187,22 @@ impl MarginCollapseInfo {
     /// Adds the child's potentially collapsible block-start margin to the current margin state and
     /// advances the Y offset by the appropriate amount to handle that margin. Returns the amount
     /// that should be added to the Y offset during block layout.
-    pub fn advance_block_start_margin(&mut self, child_collapsible_margins: &CollapsibleMargins) -> Au {
+    pub fn advance_block_start_margin(&mut self, child_collapsible_margins: &CollapsibleMargins)
+                                      -> Au {
         match (self.state, *child_collapsible_margins) {
-            (MarginCollapseState::AccumulatingCollapsibleTopMargin, CollapsibleMargins::None(block_start, _)) => {
+            (MarginCollapseState::AccumulatingCollapsibleTopMargin,
+             CollapsibleMargins::None(block_start, _)) => {
                 self.state = MarginCollapseState::AccumulatingMarginIn;
                 block_start
             }
-            (MarginCollapseState::AccumulatingCollapsibleTopMargin, CollapsibleMargins::Collapse(block_start, _)) => {
+            (MarginCollapseState::AccumulatingCollapsibleTopMargin,
+             CollapsibleMargins::Collapse(block_start, _)) => {
                 self.block_start_margin.union(block_start);
                 self.state = MarginCollapseState::AccumulatingMarginIn;
                 Au(0)
             }
-            (MarginCollapseState::AccumulatingMarginIn, CollapsibleMargins::None(block_start, _)) => {
+            (MarginCollapseState::AccumulatingMarginIn,
+             CollapsibleMargins::None(block_start, _)) => {
                 let previous_margin_value = self.margin_in.collapse();
                 self.margin_in = AdjoiningMargins::new();
                 previous_margin_value + block_start