[PCCT-SideSheet] Fix shadow flicker when minimizing side sheet

(cherry picked from commit 26bbdf1)

Bug: 1450756
Change-Id: I4141bc7d8f04bec99f6fa7f3fc2f5b5684905ff2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4581864
Commit-Queue: Zach Katz <katzz@google.com>
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1152706}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4590692
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/branch-heads/5790@{#381}
Cr-Branched-From: 1d71a33-refs/heads/main@{#1148114}

chrome/android/java/src/org/chromium/chrome/browser/customtabs/features/partialcustomtab/PartialCustomTabSideSheetStrategy.java

@@ -217,7 +217,9 @@ private void onMaximizeEnd() {
                 }
                 initializeSize();
                 if (shouldDrawDividerLine()) drawDividerLine();
-                updateShadowOffset();
+                // We have a delay before showing the resized web contents so it has to be done
+                // for the shadow as well.
+                new Handler().postDelayed(this::updateShadowOffset, 20);
                 maybeInvokeResizeCallback();
             });
         }