The previous FlowLayout._do_layout walked each thumb summing
`widget.width() + THUMB_SPACING` and wrapped on `x + item_w >
self.width()`. This was vulnerable to two issues that conspired to
produce the "grid collapses by a column when switching to a post"
bug:
1. **Per-widget width drift**: ThumbnailWidget calls
`setFixedSize(THUMB_SIZE, THUMB_SIZE)` in __init__, capturing the
constant at construction time. If `THUMB_SIZE` is later mutated
via `_apply_settings` (main_window.py:2953 writes
`grid_mod.THUMB_SIZE = new_size`), existing thumbs keep their old
fixed size while new ones (e.g. from infinite-scroll backfill via
`append_posts`) get the new value. Mixed widths break the
width-summing wrap loop.
2. **Off-by-one in the columns property**: `w // (THUMB_SIZE +
THUMB_SPACING)` overcounted by 1 at column boundaries because it
omitted the leading THUMB_SPACING margin. A row that fits N
thumbs needs `THUMB_SPACING + N * step` pixels, not `N * step`.
For width=1135 with step=188, the formula returned 6 columns
while `_do_layout` only fit 5 — the two diverged whenever the
grid sat in the boundary range.
Both are fixed by using a deterministic position formula:
cols = max(1, (width - THUMB_SPACING) // step)
for each thumb i:
col = i % cols
row = i // cols
x = THUMB_SPACING + col * step
y = THUMB_SPACING + row * step
The layout is now a function of `self.width()` and the constants
only — no per-widget reads, no width-summing accumulator. The
columns property uses the EXACT same formula so callers (e.g.
main_window's keyboard Up/Down nav step) always get the value the
visual layout actually used.
Standardizing on the constant means existing thumbs that were
created with an old `THUMB_SIZE` value still position correctly
(they sit in the cells positioned by the new step), and any future
mutation of THUMB_SIZE only affects newly-created thumbs without
breaking the layout of the surviving ones.
Affects all three tabs (Browse / Bookmarks / Library) since they
all use ThumbnailGrid from grid.py.
Verification:
- Phase A test suite (16 tests) still passes
- Popout state machine tests (65 tests) still pass
- Total: 81 / 81 automated tests green
- Imports clean
- Manual: open the popout to a column boundary (resize window
width such that the grid is exactly N columns wide), switch
between posts — column count should NOT flip to N-1 anymore.
Also verify keyboard Up/Down nav steps by exactly the column
count visible on screen (was off-by-one before at boundaries).
pax
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