Reddit Multi Column: fix residual iOS stacked-column flash on post tap (v0.3.24)

[heyflorin]

Problem

On iPad/iOS Safari, tapping a post sometimes un-grids the multi-column feed into a single stacked column for a moment before the post page renders — the symptom v0.3.23 set out to fix, still leaking through intermittently.

Root cause

v0.3.23 keeps the lingering previous feed hidden, but recognises it by reference equality (found === priorFeed). Reddit's router navigates to a post with a pushState immediately followed by a replaceState to the canonical permalink (the URL with the title slug) — two path changes in the same task. onNavigate nulls parent on the first change, so the second one captures priorFeed = null:

priorFeed = (parent && parent.isConnected) ? parent : null; // parent already null → null

The re-search then re-finds the still-visible gridded feed, found === priorFeed is found === null → false, so it takes the revealFeed branch and standDown() un-grids the feed while it is still visible = the stacked flash. Desktop swaps too fast to ever show the un-gridded frame; the slow iOS/iPadOS SPA swap leaves it on screen. This is why it was intermittent.

Fix

Mark every feed we grid with a durable __rmcGridded flag and decide suppression from that flag (read before standDown()) instead of the fragile priorFeed reference. A gridded node re-found on a post-detail page is, by definition, the previous feed lingering through the swap, so it stays hidden no matter how priorFeed was lost. priorFeed is kept only as a secondary signal.

The behavioural change is 4 lines; the rest is the changelog/comment block and a version bump to 0.3.24.

// makeLayout (grid path)
parent.__rmcGridded = true;

// engageFeed (stand-down branch)
const wasGridded = !!found.__rmcGridded || found === priorFeed;
standDown();
if (wasGridded && isPostDetail()) { suppressed = true; hideFeed(); }
else { suppressed = false; revealFeed(); }

The change only alters the isPostDetail()-true branch, so portrait/too-narrow rotation, profile, and search stand-downs are byte-for-byte the prior revealFeed behavior and cannot regress.

Verification

Built a headless-browser harness (Playwright + Chromium) that serves a mock Reddit SPA, injects the real userscript, grids the feed, then simulates the post tap with the slow iOS-style content swap. A MutationObserver (fires after each synchronous script batch — the state about to paint) plus a rAF sampler record any frame where the original feed is effectively visible and un-gridded.

Scenario	Before (0.3.23)	After (0.3.24)
single-nav (control)	ok	ok
single-nav + churn	ok	ok
double-nav (push→replace)	FLASH 8/8 @ op 0.98	ok
double-nav + churn	FLASH 8/8	ok
double-nav + slow swap 250ms	FLASH 8/8	ok
triple-nav (push + 2× replace)	FLASH 8/8	ok
profile-comments mixed feed	ok	ok
double-nav then BACK	FLASH 8/8	ok — re-grids & reveals (op 1.00, 8/8)

Back-navigation correctly re-grids and reveals the feed; the genuine profile-comments mixed feed is unaffected; no page errors.

🤖 Generated with Claude Code

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