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+# Responsive Design Mode Architecture
+
+## Context
+
+You have a single browser tab that has visited several pages, and now has a
+history that looks like, in oldest to newest order:
+
+1. https://newsblur.com
+2. https://mozilla.org (← current page)
+3. https://convolv.es
+
+## Opening RDM During Current Firefox Session
+
+When opening RDM, the browser tab's history must preserved.  Additionally, we
+strive to preserve the exact state of the currently displayed page (effectively
+any in-page state, which is important for single page apps where data can be
+lost if they are reloaded).
+
+This seems a bit convoluted, but one advantage of this technique is that it
+preserves tab state since the same tab is reused.  This helps to maintain any
+extra state that may be set on tab by add-ons or others.
+
+1. Create a temporary, hidden tab to load the tool UI.
+2. Mark the tool tab browser's docshell as active so the viewport frame is
+   created eagerly and will be ready to swap.
+3. Create the initial viewport inside the tool UI.
+4. Swap tab content from the regular browser tab to the browser within the
+   viewport in the tool UI, preserving all state via
+   `gBrowser._swapBrowserDocShells`.
+5. Force the original browser tab to be non-remote since the tool UI must be
+   loaded in the parent process, and we're about to swap the tool UI into
+   this tab.
+6. Swap the tool UI (with viewport showing the content) into the original
+   browser tab and close the temporary tab used to load the tool via
+   `swapBrowsersAndCloseOther`.
+7. Start a tunnel from the tool tab's browser to the viewport browser
+   so that some browser UI functions, like navigation, are connected to
+   the content in the viewport, instead of the tool page.
+
+## Closing RDM During Current Firefox Session
+
+To close RDM, we follow a similar process to the one from opening RDM so we can
+restore the content back to a normal tab.
+
+1. Stop the tunnel between outer and inner browsers.
+2. Create a temporary, hidden tab to hold the content.
+3. Mark the content tab browser's docshell as active so the frame is created
+   eagerly and will be ready to swap.
+4. Swap tab content from the browser within the viewport in the tool UI to the
+   regular browser tab, preserving all state via
+   `gBrowser._swapBrowserDocShells`.
+5. Force the original browser tab to be remote since web content is loaded in
+   the child process, and we're about to swap the content into this tab.
+6. Swap the content into the original browser tab and close the temporary tab
+   used to hold the content via `swapBrowsersAndCloseOther`.
+
+## Session Restore
+
+When restarting Firefox and restoring a user's browsing session, we must
+correctly restore the tab history.  If the RDM tool was opened when the session
+was captured, then it would be acceptable to either:
+
+* A: Restore the tab content without any RDM tool displayed **OR**
+* B: Restore the RDM tool the tab content inside, just as before the restart
+
+We currently follow path A (no RDM after session restore), which seems more in
+line with how the rest of DevTools currently functions after restore.  To do so,
+we watch for `beforeunload` events on the tab at shutdown and quickly exit RDM
+so that session restore records only the original page content during its final
+write at shutdown.