Always call WasShown() before trying to paint.

chrome/browser/ui/browser_instant_controller.cc

Index: chrome/browser/ui/browser_instant_controller.cc
diff --git a/chrome/browser/ui/browser_instant_controller.cc b/chrome/browser/ui/browser_instant_controller.cc
index dc58960c5b0c7f73a66a246cab065c3086828af6..293d9f51325d7e0fbae15bb8dfb9ab3427a26ab1 100644
--- a/chrome/browser/ui/browser_instant_controller.cc
+++ b/chrome/browser/ui/browser_instant_controller.cc
@@ -78,6 +78,7 @@ bool BrowserInstantController::OpenInstant(WindowOpenDisposition disposition) {
 
 void BrowserInstantController::ShowInstant() {
   TabContents* preview = instant_->GetPreviewContents();
+  preview->web_contents()->WasShown();

[sky, 2012/08/20 16:29:31]

Any reason you're not putting this in InstantController?

[sreeram_google.com, 2012/08/20 16:36:45]

No particular reason. Historically, this code has been here, in Browser land. It
seems reasonable that the Browser should operate on the active tab.
InstantController only uses the active tab for things like pointer comparison,
extracting profile and passing on to other methods. It doesn't directly
"operate" on it. Let me know if you'd like to change that.

[sreeram, 2012/08/20 16:46:25]

I forgot to add that, since the Browser operates on the active tab, it seems
reasonable to have it operate on the preview as well (since the Hide() on one
and Show() on the other go hand in hand).

   browser_->window()->ShowInstant(preview);
 
   content::NotificationService::current()->Notify(
@@ -88,10 +89,12 @@ void BrowserInstantController::ShowInstant() {
   // TODO(beng): Investigate if we can avoid this and instead rely on the
   //             visibility of the WebContentsView.
   chrome::GetActiveWebContents(browser_)->WasHidden();
-  preview->web_contents()->WasShown();
 }
 
 void BrowserInstantController::HideInstant() {
+  if (chrome::GetActiveWebContents(browser_))
+    chrome::GetActiveWebContents(browser_)->WasShown();

[sky, 2012/08/20 16:29:31]

How come you're moving this?

[sreeram_google.com, 2012/08/20 16:36:45]

Same reason as before. When we call browser_->window()->HideInstant(), it may
try to paint the active tab and ask for its backing store, which might trigger
the DCHECK() since it's hidden. Hence, make the active tab visible first.

+
   browser_->window()->HideInstant();
 
   content::NotificationService::current()->Notify(
@@ -99,8 +102,6 @@ void BrowserInstantController::HideInstant() {
       content::Source<InstantController>(instant()),
       content::NotificationService::NoDetails());
 
-  if (chrome::GetActiveWebContents(browser_))
-    chrome::GetActiveWebContents(browser_)->WasShown();
   if (TabContents* preview = instant_->GetPreviewContents())
     preview->web_contents()->WasHidden();
 }