Fix random problem selection by difficulty

[Ericko00o]

Explicit random selection currently shares the spaced-review recommendation policy. When multiple problems are overdue, repeated clicks can cycle through those problems indefinitely, excluding unseen problems and selecting reviews outside the chosen difficulties.

Separate explicit random selection from automatic recommendations. Random draws uniformly sample the problems matching the selected difficulties and topic filter, excluding the current problem whenever an alternative exists. Initial recommendations retain overdue-review priority across the broader review pool. Random selections also display the problem's difficulty.

Add regression tests for unseen-problem eligibility, difficulty filtering, current-problem exclusion, single-problem and empty pools, and the distinction between filtered practice and broader review recommendations. Update the lobby's expected selection text and add browser coverage for random selection, page restoration, and difficulty changes.

Validation: node --test tests/browser/problem-picker.test.js tests/browser/lobby.test.js completed with 43 tests passing, no failures, and 112 browser tests skipped because Playwright Chromium was unavailable. The JavaScript syntax check and git diff --cached --check passed. The required ./scripts/test.sh gate has not been run, and browser interaction remains unverified locally.

Closes #210

---

Summary by cubic

Fixes #210 by separating explicit random selection from the spaced-review recommendation policy. Random draws now sample uniformly from the problems matching the selected difficulties and topic filter instead of cycling through overdue reviews, which previously left unseen problems unreachable.

• Random draws exclude the current problem whenever an alternative exists and show the chosen difficulty.
• Recommendations keep overdue-review priority across the broader review pool.
• Adds regression tests for unseen-problem eligibility, difficulty filtering, exclusion, narrowed-practice pools, and empty-pool handling.

^{Written for commit ac3968c. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 4 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. When an issue isn't valid or won't be fixed in this PR, reply in its thread with the reason and then resolve the thread. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="web/app.js">

<violation number="1" location="web/app.js:1028">
P2: Topic-filter redraws in random mode can select the current problem again because `filterSelectionChanged()` clears `avoidedProblem` before `recommend()`. Preserve the current card as the exclusion on random-mode redraws.</violation>
</file>

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[cubic-dev-ai]

P2: Topic-filter redraws in random mode can select the current problem again because filterSelectionChanged() clears avoidedProblem before recommend(). Preserve the current card as the exclusion on random-mode redraws.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. When an issue isn't valid or won't be fixed in this PR, reply in its thread with the reason and then resolve the thread. At web/app.js, line 1028:

<comment>Topic-filter redraws in random mode can select the current problem again because `filterSelectionChanged()` clears `avoidedProblem` before `recommend()`. Preserve the current card as the exclusion on random-mode redraws.</comment>

<file context>
@@ -1021,6 +1025,7 @@ function recommend(note = "") {
     undefined,
     avoidedProblem,
     cards,
+    pickerMode,
   );
   // Nothing to offer is still an answer, and it has to go through `setProblem`
</file context>

[jserv]

A level change is not an explicit random draw, but this switches it to uniform selection anyway. With a passed Hard problem and unseen Hard ones, checking Hard and unchecking the current level can now offer the passed problem first. That undoes "a problem already passed is not what gets recommended next", and the description does not mention it. If the goal is only to stop reviews crossing the selected levels, keep "recommend" here and restrict the review pool to the selected difficulties instead.

[jserv]

pickerMode is set to "random" and never goes back to "recommend", so after one click every later recommend() draws uniformly: filterSelectionChanged(), sign-in, report deletion and every settle(). Concrete case: click Random and get X, finish X, press Back. The bfcache restore refetches reports that now record X as passed, yet roll, avoidedProblem and the pool are unchanged, so settle() offers X again (and silently drops the reason keptDraw is cleared there). Reset to "recommend" wherever avoidedProblem is reset to undefined, and in settle() whenever the reports differ from the ones the random draw was made from (track them the way keptDraw does).

[jserv]

"Editing begins:" reads like an editor marker. A comment here should say why random mode skips the review sentence, e.g. that a random draw is never a review and the level is named because it may differ from the suggested one.

[jserv]

The difficulty half of this test only runs after the Random click has already set pickerMode = "random", so deleting the assignment in the level handler still passes. It also checks only the level, not that the note lacks Review due. Exercise the level change from a fresh lobby with overdue reviews, as a separate test, and assert both.

[jserv]

Check CONTRIBUTING.md carefully and enforce the rules. In particular, the manner for writing git commit messages.