RenderText: handle center-aligned text in UpdateCachedBoundsAndOffset()

ui/gfx/render_text.cc

Index: ui/gfx/render_text.cc
diff --git a/ui/gfx/render_text.cc b/ui/gfx/render_text.cc
index fb7364b66f2b538f5733867b11afc11330755c6d..62ed10bb560d03e89abd1126e1a314ed5aeea49c 100644
--- a/ui/gfx/render_text.cc
+++ b/ui/gfx/render_text.cc
@@ -1342,9 +1342,11 @@ void RenderText::UpdateCachedBoundsAndOffset() {
     // Reduce the pan offset to show additional overflow text when the display
     // width increases.
     const int negate_rtl = horizontal_alignment_ == ALIGN_RIGHT ? -1 : 1;

[msw, 2014/07/11 22:33:04]

nit: consider combining the two scale factors.

-    const int offset = negate_rtl * display_offset_.x();
+    const int double_center = horizontal_alignment_ == ALIGN_CENTER ? 2 : 1;

[msw, 2014/07/11 22:33:04]

nit: consider |scale_center|

+    const int offset = negate_rtl * double_center * display_offset_.x();
     if (display_width > (content_width + offset)) {

[msw, 2014/07/11 22:33:04]

I don't think this condition will capture all the necessary cases. Consider a
10px display width, with a 50px text width, offset by -10px. Now, increase the
display width to 21px. This condition will not fire, because 21 < 50 + 2 *
(-10),  but that'll leave some extra empty space on the right...

I don't understand the |double_center| scale factor for |offset| and wonder if
we need to consider the value of |GetAlignmentOffset|. Can you take a closer
look and more clearly explain what's going on (especially if I'm incorrect)?

[mohsen, 2014/07/14 20:57:01]

In your example, no extra empty space will be shown on the right. Suppose center
of textfield is fixed at point 0. If |display_width| is 21 and |content_width|
is 50, textfield will cover [-10.5, 10.5] and text will cover [-25, 25], when
there is no offset. Now adding offset of -10 shifts the text 10 pixels to left,
so it will cover [-35, 15] which is still covering the entire textfield.

As for |double_center| scale factor, maybe it's best to think of it as follows:
in case of center-aligned text, if we divide both text and textfield in half and
only consider their right halves, it's like they're left-aligned and all
calculations for left-aligned texts would work.

The above explanation works when offset is less than zero. If offset is greater
than zero, then we should use calculations for right-aligned texts. I haven't
handled this case here (my mistake) and adding it will make this code more
complicated. Instead, I'm going to switch to using SetDisplayOffset() function
which handles all these cases.

[msw, 2014/07/16 01:41:43]

I think this is wrong, the alignment offset 
Ah, I didn't account for GetAlignmentOffset.

-      delta_x = negate_rtl * (display_width - (content_width + offset));
+      delta_x = negate_rtl * (display_width - (content_width + offset)) /
+          double_center;
     }
   }