Keep UI process session history authoritative across overlapping
fallback loads and traversals. WebContent can finish a superseded
history load with a live document matching the UI seed URL while its
local step, document state id, and Navigation API keys still describe
a temporary partial list.
Reconstruct the current entry around the UI-owned list in that case.
This avoids making the UI process adopt WebContent's incomplete
snapshot.
Track UI-started fallback loads by URL so unrelated navigations cannot
consume the pending seed state. Resolve deferred WebDriver completions
through the view registry so callbacks queued before a process swap do
not touch a destroyed view.
Add WebDriver coverage that waits for explicit UI/WebContent history
convergence after the relevant document events. The waits poll
observable history state instead of depending on timing.
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