split view: Anticipate that dragging may begin in a snap area

The present CL modifies code that was based on a bad assumption: that
dragging always begins with one of the kFrom* window dragging states.

Test: manual
Fixed: 1024513
Change-Id: Id515edfc9344af8a86ca49b14583af244b248652
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918034Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715993}

ash/wm/splitview/split_view_drag_indicators.cc

@@ -181,11 +181,13 @@ class SplitViewDragIndicators::RotatedImageLabelView : public views::View {
       return;
     }
 
-    // When dragging begins, set the text and fade in with an indicator.
+    // Set the text according to |can_dragged_window_be_snapped|.
+    SetLabelText(l10n_util::GetStringUTF16(
+        can_dragged_window_be_snapped ? IDS_ASH_SPLIT_VIEW_GUIDANCE
+                                      : IDS_ASH_SPLIT_VIEW_CANNOT_SNAP));
+
+    // When dragging begins (without a snap preview), fade in with an indicator.
     if (previous_window_dragging_state == WindowDraggingState::kNoDrag) {
-      SetLabelText(l10n_util::GetStringUTF16(
-          can_dragged_window_be_snapped ? IDS_ASH_SPLIT_VIEW_GUIDANCE
-                                        : IDS_ASH_SPLIT_VIEW_CANNOT_SNAP));
       DoSplitviewOpacityAnimation(
           layer(), SPLITVIEW_ANIMATION_TEXT_FADE_IN_WITH_HIGHLIGHT);
       return;

ash/wm/splitview/split_view_highlight_view.cc

@@ -206,6 +206,9 @@ void SplitViewHighlightView::OnWindowDraggingStateChanged(
     return;
   }
 
+  // Set the color according to |can_dragged_window_be_snapped|.
+  SetColor(can_dragged_window_be_snapped ? SK_ColorWHITE : SK_ColorBLACK);
+
   if (preview_position != SplitViewController::NONE) {
     DoSplitviewOpacityAnimation(
         layer(), is_right_or_bottom_ != IsPhysicalLeftOrTop(preview_position)
@@ -236,12 +239,11 @@ void SplitViewHighlightView::OnWindowDraggingStateChanged(
   }
 
   // The drag just started, and not in a snap area. If |previews_only|, there is
-  // nothing to do. Else set the color of the indicators and fade them in.
+  // nothing to do. Else fade in.
   DCHECK_EQ(SplitViewDragIndicators::WindowDraggingState::kNoDrag,
             previous_window_dragging_state);
   DCHECK_EQ(0.f, layer()->opacity());
   if (!previews_only) {
-    SetColor(can_dragged_window_be_snapped ? SK_ColorWHITE : SK_ColorBLACK);
     DoSplitviewOpacityAnimation(layer(), SPLITVIEW_ANIMATION_HIGHLIGHT_FADE_IN);
     return;
   }