Bug 293504. Make MEW/max-width calculations in scrollframes consistent with regular reflow about how we handle the scrollbar width. r+sr=dbaron,a=chofmann

layout/generic/nsBlockReflowContext.cpp

@@ -549,6 +549,9 @@ nsBlockReflowContext::ReflowBlock(const nsRect&       aSpace,
     nscoord oldComputedWidth = aFrameRS.mComputedWidth;
 
     aFrameRS.availableWidth = NS_UNCONSTRAINEDSIZE;
+    // XXX Is this really correct? This means we don't compute the
+    // correct maximum width if the element's width is determined by
+    // its 'width' style
     aFrameRS.mComputedWidth = NS_UNCONSTRAINEDSIZE;
     rv = mFrame->Reflow(mPresContext, mMetrics, aFrameRS, aFrameReflowStatus);
 

layout/generic/nsGfxScrollFrame.cpp

@@ -434,19 +434,28 @@ nsHTMLScrollFrame::TryLayout(ScrollReflowState* aState,
     // their content box can be squeezed down to 0, as they will either create
     // a scrollbar so that content of the scrollframe will not leak out or it
     // will cut the content at the frame boundaries.
-    aState->mMaxElementWidth = vScrollbarActualWidth +
-      aState->mReflowState.mComputedPadding.LeftRight() +
-      aState->mReflowState.AdjustIntrinsicMinContentWidthForStyle(0);
+    
+    // The width of the vertical scrollbar comes out of the budget for the
+    // content width (see above where we include the scrollbar width before
+    // we call ComputeInsideBorderSize, which overrides the given
+    // width with the style computed width if there is one). So allow
+    // the vertical scrollbar width to be overridden by style information
+    // here, too.
+    nscoord minContentWidth =
+      aState->mReflowState.AdjustIntrinsicMinContentWidthForStyle(vScrollbarActualWidth);
+    aState->mMaxElementWidth = minContentWidth +
+      aState->mReflowState.mComputedPadding.LeftRight();
     // borders get added on the way out of Reflow()
   }
   if (aKidMetrics.mFlags & NS_REFLOW_CALC_MAX_WIDTH) {
+    // We need to do what we did above: include the vertical scrollbar width in the
+    // content width before applying style.
     nscoord kidMaxWidth = aKidMetrics.mMaximumWidth;
     if (kidMaxWidth != NS_UNCONSTRAINEDSIZE) {
       nscoord kidContentMaxWidth = kidMaxWidth -
-        aState->mReflowState.mComputedPadding.LeftRight();
+        aState->mReflowState.mComputedPadding.LeftRight() + vScrollbarActualWidth;
       NS_ASSERTION(kidContentMaxWidth >= 0, "max-width didn't include padding?");
-      kidMaxWidth = vScrollbarActualWidth +
-        aState->mReflowState.mComputedPadding.LeftRight() +
+      kidMaxWidth = aState->mReflowState.mComputedPadding.LeftRight() +
         aState->mReflowState.AdjustIntrinsicContentWidthForStyle(kidContentMaxWidth);
     }
     aState->mMaximumWidth = kidMaxWidth;

layout/generic/nsHTMLReflowState.cpp

@@ -386,9 +386,12 @@ nsHTMLReflowState::AdjustIntrinsicMinContentWidthForStyle(nscoord aWidth) const
   if (eStyleUnit_Percent == widthUnit) {
     aWidth = 0;
   } else if (eStyleUnit_Coord == widthUnit) {
-    NS_ASSERTION(NS_UNCONSTRAINEDSIZE != mComputedWidth,
-                 "Should be a computed width here");
-    aWidth = mComputedWidth;
+    // Sometimes we can get an unconstrained size here because we're
+    // computing the maximum-width. Although it doesn't seem right
+    // for max-width computation to change our computed width.
+    if (NS_UNCONSTRAINEDSIZE != mComputedWidth) {
+      aWidth = mComputedWidth;
+    }
   }
 
   nsStyleUnit maxWidthUnit = mStylePosition->mMaxWidth.GetUnit();
@@ -415,9 +418,12 @@ nsHTMLReflowState::AdjustIntrinsicContentWidthForStyle(nscoord aWidth) const
 {
   nsStyleUnit widthUnit = mStylePosition->mWidth.GetUnit();
   if (eStyleUnit_Coord == widthUnit) {
-    NS_ASSERTION(NS_UNCONSTRAINEDSIZE != mComputedWidth,
-                 "Should be a computed width here");
-    aWidth = mComputedWidth;
+    // Sometimes we can get an unconstrained size here because we're
+    // computing the maximum-width. Although it doesn't seem right
+    // for max-width computation to change our computed width.
+    if (NS_UNCONSTRAINEDSIZE != mComputedWidth) {
+      aWidth = mComputedWidth;
+    }
   }
 
   nsStyleUnit maxWidthUnit = mStylePosition->mMaxWidth.GetUnit();