faster painting of empty inlines, prep for positioning

perf-2 gets another 4-5ms by skipping the BoxBuilder stuff when
there aren't any inlines to paint in a paragraph

this is prep for relative positioning: now that inlines and items
get position information before paint, the postprocess step can
easily offset inlines and blocks according to `position: relative`
and the 4 side properties. this also enables
`display: inline-block` because the relative position of an inline-
block can't be known until after bidi reordering.

src/flow.ts

@@ -1321,15 +1321,16 @@ export class IfcInline extends Inline {
   public paragraph: Paragraph;
   private analysis: number;
 
-  static ANALYSIS_HAS_TEXT        = 1 << 0;
-  static ANALYSIS_WRAPS           = 1 << 1;
-  static ANALYSIS_WS_COLLAPSES    = 1 << 2;
-  static ANALYSIS_HAS_INLINES     = 1 << 3;
-  static ANALYSIS_HAS_BREAKS      = 1 << 4;
-  static ANALYSIS_IS_COMPLEX_TEXT = 1 << 5;
-  static ANALYSIS_HAS_SOFT_HYPHEN = 1 << 6;
-  static ANALYSIS_HAS_FLOATS      = 1 << 7;
-  static ANALYSIS_HAS_NEWLINES    = 1 << 8;
+  static ANALYSIS_HAS_TEXT          = 1 << 0;
+  static ANALYSIS_WRAPS             = 1 << 1;
+  static ANALYSIS_WS_COLLAPSES      = 1 << 2;
+  static ANALYSIS_HAS_INLINES       = 1 << 3;
+  static ANALYSIS_HAS_BREAKS        = 1 << 4;
+  static ANALYSIS_IS_COMPLEX_TEXT   = 1 << 5;
+  static ANALYSIS_HAS_SOFT_HYPHEN   = 1 << 6;
+  static ANALYSIS_HAS_FLOATS        = 1 << 7;
+  static ANALYSIS_HAS_NEWLINES      = 1 << 8;
+  static ANALYSIS_HAS_PAINTED_SPANS = 1 << 9;
 
   constructor(style: Style, text: string, children: InlineLevel[], attrs: number) {
     super(0, text.length, style, children, Box.ATTRS.isAnonymous | attrs);
@@ -1382,6 +1383,9 @@ export class IfcInline extends Inline {
         if (!isWsPreserved(box.style.whiteSpace)) {
           this.analysis |= IfcInline.ANALYSIS_WS_COLLAPSES;
         }
+        if (box.style.backgroundColor.a !== 0 || box.style.hasBorder()) {
+          this.analysis |= IfcInline.ANALYSIS_HAS_PAINTED_SPANS;
+        }
         stack.unshift(...box.children);
       } else if (box.isBreak()) {
         this.analysis |= IfcInline.ANALYSIS_HAS_BREAKS;
@@ -1433,6 +1437,7 @@ export class IfcInline extends Inline {
   doTextLayout(ctx: LayoutContext) {
     if (this.hasText() || this.hasFloats()) {
       this.paragraph.createLineboxes(ctx);
+      this.paragraph.positionItems(ctx);
     }
   }
 
@@ -1472,6 +1477,10 @@ export class IfcInline extends Inline {
     return this.analysis & IfcInline.ANALYSIS_HAS_NEWLINES;
   }
 
+  hasPaintedSpans() {
+    return this.analysis & IfcInline.ANALYSIS_HAS_PAINTED_SPANS;
+  }
+
   assignContainingBlocks(ctx: LayoutContext) {
     this.containingBlock = ctx.lastBlockContainerArea;
   }