responsive.horizontal overflow

responsive.horizontal-overflow

Rule ID: responsive.horizontal-overflow Severity: WARN Category: responsive Target Standards: W3C CSS Overflow Module Level 3, Google Web Vitals (Preventing Unintended Layout Shifts), iOS WebKit Touch Gesture Chaining Guidelines

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1. Overview & Core Invariant

Warns when unconstrained overflow-x-scroll is declared without fluid width boundary or dynamic auto-scrolling

Core Invariant:

"Horizontal scroll containers on mobile baseline must declare fluid boundaries ('w-full', 'max-w-full') and use dynamic scrolling ('overflow-x-auto') rather than forced permanent scrollbar rails ('overflow-x-scroll')."

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2. Technical Grounding & Engine Realities

Declaring 'overflow-x-scroll' directly on mobile baseline forces WebKit and Chromium browsers to render a permanent, unyielding scrollbar rail even when content fits within the viewport.

Furthermore, when horizontal scrolling lacks explicit boundary containment ('w-full' or 'max-w-full'), touch drag events can bleed into root document scrolling, causing disorienting horizontal page wobble.

Using 'overflow-x-auto w-full' ensures content only scrolls when overflowing, preserves natural gesture chaining, and prevents horizontal page wobble.

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3. Vulnerability & Risk Taxonomy

Risk Vector	Severity	Impact
Forced Permanent Scrollbar Rails	MEDIUM	Rigid horizontal scrollbar tracks clutter compact mobile viewports even when content fits completely.
Broken Vertical Touch Gesture Chaining	LOW	Users attempting vertical swipe navigation get trapped inside unconstrained horizontal scroll containers.

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4. Non-Compliant Code Patterns (Bad Examples)

TSX (Unbounded overflow-x-scroll forcing persistent scrollbar rail):

<div className="overflow-x-scroll">
  <div className="flex gap-4">
    <div className="p-4 bg-card">Item 1</div>
  </div>
</div>

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5. Compliant Implementation Patterns (Good Examples)

TSX (Fluid container using dynamic overflow-x-auto):

<div className="w-full overflow-x-auto">
  <div className="flex gap-4 min-w-max">
    <div className="p-4 bg-card">Item 1</div>
  </div>
</div>

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6. Detection & Verification Pipeline (How The Rule Evaluates Code)

This rule evaluates source code through the standard AST inspection pipeline:

flowchart TD
    Node["AST Node (Astro / TSX element)"] --> Inspect["1. Inspect Element & Attributes"]
    Inspect --> Invariant{"2. Evaluate Rule Invariant"}
    Invariant -- "Compliant" --> Safe["Pass"]
    Invariant -- "Non-Compliant" --> IgnoreCheck{"3. Check charites:ignore directive"}
    IgnoreCheck -- "Ignored" --> Safe
    IgnoreCheck -- "Not Ignored" --> Diag["4. Emit Diagnostic: responsive.horizontal-overflow"]

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Step-by-Step Evaluation:

1. AST Traversal: Visits normalized ir.Node structures during AST streaming.
2. Invariant Check: Compares node attributes against rule invariants.
3. Directive Suppression Check: Honors inline charites:ignore responsive.horizontal-overflow directives.
4. Diagnostic Emission: Emits compiler-grade diagnostic on invariant violation.

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7. Verification & Test Harness (How The Test Works: 1-SSOT Tri-Corpus)

This rule is rigorously tested and validated across the canonical 1-SSOT Tri-Corpus in tests/correctness/responsive.horizontal-overflow/:

flowchart TD
    subgraph GoldenCorpus ["1-SSOT Tri-Corpus Test Matrix for responsive.horizontal-overflow"]
        subgraph P ["Positive Corpus (tests/correctness/responsive.horizontal-overflow/positive/)"]
            P1["P1: Obvious Direct Violation"]
            P2["P2: Indirect / Variant Concatenation"]
            P3["P3: Helper / clsx / cn Wrapper"]
            P4["P4: Deeply Nested Elements"]
            P5["P5: Aliased Imports / Re-exports"]
        end
        subgraph N ["Negative Corpus (tests/correctness/responsive.horizontal-overflow/negative/)"]
            N1["N1: Valid Design Tokens"]
            N2["N2: Explicit charites:ignore Directive"]
            N3["N3: Third-Party / Vendor Components"]
            N4["N4: Clean Semantic HTML"]
            N5["N5: Untokenized Custom Values (Banana Test)"]
        end
        subgraph A ["Adversarial Corpus (tests/correctness/responsive.horizontal-overflow/adversarial/)"]
            A1["A1: Template Literal Interpolations"]
            A2["A2: Ternary Conditional Expressions"]
            A3["A3: Spread Properties & Dynamic Overrides"]
            A4["A4: Dynamic Object Class Syntax"]
            A5["A5: Shadowed Variable Identifiers"]
            A6["A6: Nested Closures & HOC Wrappers"]
            A7["A7: Obfuscated Classes & Cyclic Tokens"]
        end
    end

    P --> TestRunner["Automated Runner (rule_test.go)"]
    N --> TestRunner
    A --> TestRunner
    TestRunner --> Gates["Quality Gates: Zero Panic, Zero False-Positive, Zero Bypass"]

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• Positive Fixtures (P1-P5): Verified to trigger diagnostics at exact lines and column spans.
• Negative Fixtures (N1-N5): Verified to produce zero diagnostics on valid tokens and legitimate exemptions.
• Adversarial Fixtures (A1-A7): Verified to prevent evasion across dynamic expressions, string interpolations, and cyclic references.

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8. How to Suppress (Ignore Directives)

If this pattern is required for an intentional exception, suppress the diagnostic using the canonical Charites Rule ID:

<!-- charites:ignore responsive.horizontal-overflow intentional exception -->

// charites:ignore responsive.horizontal-overflow intentional exception

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9. Configuration Reference (charites.yaml)

rules:
  responsive.horizontal-overflow:
    severity: warn # error | warn | info | off