Addresses PR #12807 review feedback:

1. BLOCKING: Restructured E2E tests to focus on width modes
   - Changed from context-based tests (fragment/expander/container)
     to parameterized tests covering all combinations of:
     * Width modes: default (stretch/legacy) and content
     * Contexts: fragment, expander, container
   - Tests now clearly communicate what's being tested
   - Eliminated redundant test functions (6 tests → 1 parameterized + 1 count check)

2. Improved terminology in frontend code comment
   - Changed 'isUsingStretchOrDefault' to 'isUsingStretchOrLegacy'
   - Added clarifying comment about legacy behavior
   - Addresses question about what 'default' means

3. Updated test documentation
   - Removed references to 'workaround' terminology
   - Added clear explanation of width modes and bug symptoms
   - Improved comments explaining 200px threshold reasoning

Changes maintain the same test coverage but with clearer structure
and more maintainable parameterized approach.

Addresses all blocking feedback from PR #12807 review:

1. BLOCKING: Move tests into existing st_pyplot files
   - Moved regression tests from standalone files into st_pyplot.py
   - Moved test functions into st_pyplot_test.py
   - Deleted standalone st_pyplot_image_width_regression* files
   - Better for test suite performance and organization

2. BLOCKING: Add pixel width test cases
   - Added 400px width tests for all contexts (fragment, expander, container)
   - Now tests all width modes: 'stretch', 'content', and pixel values
   - Ensures conditional logic in StyledElementContainerLayoutWrapper handles all cases

3. BLOCKING: Use 'stretch' instead of 'default'
   - Changed test app to use 'stretch' directly
   - Eliminated conditional logic (can pass width value directly to st.pyplot)
   - Clearer and more aligned with Width type system

Test structure:
- 9 regression tests (3 width modes × 3 contexts)
- Parameterized test function for all combinations
- Updated element counts in existing tests (11 → 20)

All three blocking issues resolved.

…ions

Addresses three suggestions from PR #12807 review:

1. Simplify test cases (suggestion #2)
   - Reduced from 9 parameterized tests to 3 focused tests
   - Removed fragment and expander contexts (regression only in containers)
   - Eliminated parameterization that made test app hard to read
   - Simple, clear structure with named containers

2. Add deterministic width assertions (suggestion #3)
   - stretch: Container width=600, expect ~580-600px (allows padding)
   - content: Figure 6.4in @ 100 DPI = 640px, expect ~620-640px
   - pixel: Explicit width=500, expect ~480-500px
   - More stable than arbitrary >200px threshold

3. Clean up unnecessary comments (suggestion #4)
   - Removed comments that just describe next line
   - Kept meaningful comments explaining test logic and expectations
   - Improved readability

Test structure:
- 3 regression tests in containers only
- Specific figure size (6.4in x 4.8in at 100 DPI)
- Known container width for stretch test
- Deterministic assertions with reasonable margins

Note: Suggestion #1 (use_container_width check) addressed with explanation
in PR comment - current logic is correct for the new widthConfig system.

Changed from dimensional assertions to snapshot tests for better robustness:

Before:
- Asserted exact pixel dimensions (560-600px, 620-680px, 480-500px)
- Brittle due to matplotlib rendering variations across browsers
- Failed in CI with unexpected 672px for content width

After:
- Use snapshot testing like other tests in the file
- More consistent with existing test patterns
- Still catches regression (16px width would look very different)
- More maintainable and less brittle

Benefits:
- Follows existing pattern in st_pyplot_test.py
- Visual verification instead of pixel-perfect assertions
- Snapshots will clearly show if regression returns
- Works across all browsers without adjustment

Addressed suggestion #1 feedback:

The suggestion to check use_container_width field revealed a TypeScript
error - ImageList proto doesn't have useContainerWidth field (unlike
chart elements). ImageList only has deprecated 'width' field (WidthBehavior enum).

Current logic is correct:
- When widthConfig is not set → default to stretch behavior (width: 100%)
- When widthConfig.useStretch → use stretch behavior (width: 100%)
- When widthConfig has other values → use auto width

The legacy 'width' field (WidthBehavior enum) is handled by ImageList
component itself, not by the container wrapper. The container just provides
the outer width context.

Updated comment to clarify this and explain the default legacy behavior.