v0.1.12113 Aug 2026·Web APIs & DOM, DevTools, Tabs & interface

Time-travel debugging: step BACK - a debugger the reference browser does not have

Every mainstream browser debugger walks one way: over, into, out, resume. Step past the line that mattered and you reload and recreate the walk by hand. Floati's debugger now has a Back button: from any pause, step to the PREVIOUS pause - same line, same function, same variable values - and keep going backwards, or forward again. The reference browser DevTools cannot do this at all; time-travel exists only in external research tools, never in a browser's own debugger. It works because this engine is deterministic by construction: the same page walks the same pauses every run, so the past is reachable by replaying the run up to the pause before this one - no memory snapshots, no recording overhead, nothing paid when the debugger is off. Proven in the gate: travelling back lands on the byte-identical previous pause, and stepping forward again reproduces the future exactly. DevTools moves 35 to 37; overall holds at 51.

Parity with the reference browser

74overall, as of v0.1.343

Web APIs / DOM surface42

Security architecture66

GPU / graphics acceleration68

DevTools / extensions72

Networking85

Media85

Accessibility85

Real-world site compatibility99

HTML parsing100

CSS parsing + selectors100

Layout: block / flex / grid / tables / writing-modes100

Paint + visual100

JavaScript language100

JavaScript performance (JIT)100

HTML parsing100

What changed 4 changes

DevTools	Step BACK from any pause, repeatedly: the engine records the exact walk of debugger commands that led to the current pause, and stepping back abandons the run and replays it minus the last step - the deterministic engine parks at exactly the previous pause with every scope variable at its earlier value. Machine-checked in the gate: the replayed pause is byte-identical to the original (reason, function, and all variable values), stepping forward after travelling back reproduces the next pause byte-identically, and at the first pause Back honestly reports there is nowhere to go.
Tabs & interface	Wired everywhere the debugger lives: a Back button beside Over/Into/Out in the visual Debugger panel (one click walks backwards - the panel re-runs the page and the replay stops one pause earlier), and Debugger.stepBack on the WebSocket protocol transport - a method the protocol has listed as experimental for years but the reference browser never implemented; Floati answers it, so an external tool attached over the wire can step backwards.
Web APIs & DOM	Found and fixed by the new end-to-end test: on a real page, a line breakpoint could land inside an engine-injected polyfill instead of the page's own script, because every script unit shared one cumulative line table. Breakpoints and step boundaries now index only the page's own scripts (engine-injected code runs at full speed and can never pause), and the Sources panel now captures the page's script on real page loads, not just the playground. Also hardened: an engine thread exiting could abort the entire process, because the GPU filter device objected to being torn down inside thread-local cleanup - the device is now deliberately kept for the process lifetime, proven by the same end-to-end tests. Conformance gate unchanged; research note docs/time-travel-debugging.md.
Tabs & interface	Found by driving the released app itself, and worth the honesty: the visual Debugger panel's enable, stepping and breakpoint BUTTONS had never actually reached the engine - the message carrying them was silently rejected over an argument-naming mismatch and the error swallowed, so the buttons flipped their state while the engine heard nothing (the underlying debugger always worked, which is why every engine-level test stayed green). One-line fix; afterwards the complete time-travel arc was verified visually in the shipping app: enable, break on line 3, reload pauses with a=1, step (b=2), Back - and the panel shows the earlier scope again with the breakpoint dot and pause line highlighted in the Sources view.