Chromium Docs

History manipulation intervention in Chromium

Reference: PSA on blink-dev

Summary

Some pages make it difficult or impossible for the user to use the browser back button to go back to the page they came from. Pages accomplish this using redirects or by manipulating the browser history, resulting in an abusive/annoying user experience.

The history manipulation intervention mitigates such abuse by making the browser’s back button skip over pages that added history entries or redirected the user without ever getting a user activation. Note that the intervention only impacts the browser back/forward buttons and not the history.back()/forward() APIs.

Here’s an example:

  1. User is on a.com and clicks to go to b.com
  2. b.com adds a history entry using pushState or navigates the user to another page (c.com) without ever getting a user activation.
  3. If the user presses back, the browser will skip b.com and go back to a.com instead.

Spec

Because this only impacts browser UI, this is allowed by the spec, which only governs the behavior of history.back/forward. However, it might be good to spec this anyway, so that users get consistent experiences in all browsers. That work is tracked at https://github.com/whatwg/html/issues/7832

Invariants

At a high level, the intervention ensures the back/forward buttons always navigate to a page the user either navigated to or interacted with. It guarantees the following invariants:

  1. Only back/forward navigations triggered by the back/forward buttons will ever skip history entries. This ensures that the history API's behavior is unaffected.
  2. The intervention marks a history entry as skippable if the document creates another history entry without a user activation.
  3. If a document receives a user activation (before or after creating history entries), its history entry is not skippable. With an activation, the document can create many unskippable same-document history entries, until either a cross-document navigation or a back/forward occurs. Note that same-document back/forwards do not normally reset any prior user activation, but the intervention stops honoring such activations for creating new entries until a new activation is received, per https://crbug.com/1248529.
  4. All same-document history entries will have the same skippable state. When marking an entry unskippable after a user activation, this ensures that the rest of the document's entries work as well. When marking an entry as skippable, this ensures that all entries for the offending document will be skipped.
  5. Revisiting a skippable history entry does not change its skippable status, unless it receives a user activation. This ensures that history.back() will not bypass the intervention, per https://crbug.com/1121293.
  6. The intervention applies to history entries created by subframes as well. A user activation on any frame on the page is sufficient to make the entry unskippable, per https://crbug.com/953056.

Details

  1. The intervention works by setting the should_skip_on_back_forward_ui_ member for a NavigationEntryImpl object. The member is initially set to false, and it is set to true if any document in the page adds a history entry without having a user activation.
  2. NavigationController::CanGoBack() will return false if all entries are marked to be skipped on back/forward UI. On Android, pressing the back button will close the current tab and a previous tab could be shown as it would normally happen on Android when the back button is pressed from the first entry of a tab. On desktop, the back button will be enabled in the browser UI, but clicking on it will do nothing. This will allow a user to long-press the button and navigate to a skippable entry explicitly, while still protecting against the same annoying/abusive experiences this intervention is intended for. For additional context, see NavigationController::ShouldEnableBackButton() and https://crbug.com/339188522.
    • This behavior is mirrored for the forward button as well. See NavigationController::CanGoForward() and NavigationController::ShouldEnableForwardButton() for details.
  3. The oldest NavigationEntryImpl that is marked as skippable is the one that is pruned if max entry count is reached.