Proposal: `browser.tabs.TAB_ID_ASSOCIATED`

[carlosjeurissen]

Introduction

This was briefly discussed in TPAC2023 and TPAC2024. Extension action popups want to know the tabId of the tab associated to the current popup to apply actions like scripting.executeScript(), and to know which tab the extension was granted activeTab permission for.

Currently, the activeTab (tabs.query({ currentWindow: true, active: true })) is used as a guess by the extension for the associated tab id. However this is conceptually different and not guaranteed to be the same as the associated tab id.

API design

Introducing a browser.tabs.TAB_ID_ASSOCIATED constant, as proposed in the 2025-11-10 TPAC2025 meeting.

Getting the actual id could be achieve by calling browser.tabs.get(browser.tabs.TAB_ID_ASSOCIATED). This also has the benefit of being potentially supported in other extension contexts like devPanels and sidebars.

Notes

Previously a direct property browser.action.associatedTabId has been proposed. See previous discussion here:
#316

[carlosjeurissen]

This was briefly discussed during TPAC2025. A potential alternative solution was proposed by using magic tab id constants in the form of browser.tabs.TAB_ID_ASSOCIATED.

One of the potential issues with browser.tabs.TAB_ID_ASSOCIATED is some of the difficulty in feature detection. It is hard to know if a specific extension API supports the constant. In addition, there are a few context types in which this could be useful:

• action popups
• sidebar documents
• devtools panels

Once the constant is added, the only way to detect if a certain context type supports the constant would be an async call to browser.tabs.get(browser.tabs.TAB_ID_ASSOCIATED). If used directly in APIs, the API calls may break if TAB_ID_ASSOCIATED is not supported for said API or not available in the context at that moment.

As mentioned during the meeting, if browser.tabs.TAB_ID_ASSOCIATED gets added. I suggest we make it a value lower than -2 to reserve the value of -2 for a potential future browser.tabs.TAB_ID_CURRENT.

@fregante what are your thoughts on the constant approach?

[fregante]

I agree it would be great to have a generic "associated tab" synchronous constant for all contexts that are associated to an ID, perhaps including the dev tools.

Hopefully this value is also available in frames of these contexts, so if a frame is loaded in the sidebar it also has an "associated tab" (if the sidePanel is linked to a specific tab when it's opened)