fix(flex): fix rounding may leave unused space on track with flex_grow

[niklasf]

A flex track containing items with flex grow (without max width) should always be filled completely.

Before, grow_unit is always rounded down, somtimes leaving empty space on flex tracks. Adds a test case to show the issue and fixes it analogously to GRID_FR (#6255).

tests/ref_imgs/flex_grow.png before (note how pixels of the red container are visible at the end of some rows):

[github-actions]

Hi 👋, thank you for your PR!

We've run benchmarks in an emulated environment. Here are the results:

ARM Emulated 32b - lv_conf_perf32b

Scene Name	Avg CPU (%)	Avg FPS	Avg Time (ms)	Render Time (ms)	Flush Time (ms)
All scenes avg.	28	37	7	7	0

Detailed Results Per Scene

Scene Name	Avg CPU (%)	Avg FPS	Avg Time (ms)	Render Time (ms)	Flush Time (ms)
Empty screen	11	33	0	0	0
Moving wallpaper	2	33	1	1	0
Single rectangle	0	50	0	0	0
Multiple rectangles	0	35 (-2)	0	0	0
Multiple RGB images	0	39	0	0	0
Multiple ARGB images	8 (-3)	43 (+1)	0 (-1)	0 (-1)	0
Rotated ARGB images	54 (-3)	44	15 (+1)	15 (+1)	0
Multiple labels	7 (+4)	33 (-2)	0	0	0
Screen sized text	94 (-3)	47	20	20	0
Multiple arcs	40 (+9)	33	7	7	0
Containers	4 (+3)	37	0	0	0
Containers with overlay	86 (-11)	21	44	44	0
Containers with opa	10 (-7)	35 (-2)	1 (+1)	1 (+1)	0
Containers with opa_layer	21 (+3)	34 (+1)	6	6	0
Containers with scrolling	45	47 (+1)	12	12	0
Widgets demo	67 (-1)	40	16	16	0
All scenes avg.	28	37	7	7	0

ARM Emulated 64b - lv_conf_perf64b

Scene Name	Avg CPU (%)	Avg FPS	Avg Time (ms)	Render Time (ms)	Flush Time (ms)
All scenes avg.	24	38	6	6	0

Detailed Results Per Scene

Scene Name	Avg CPU (%)	Avg FPS	Avg Time (ms)	Render Time (ms)	Flush Time (ms)
Empty screen	11	33	0	0	0
Moving wallpaper	1	33	0	0	0
Single rectangle	0	50	0	0	0
Multiple rectangles	0	46	0	0	0
Multiple RGB images	0	39	0	0	0
Multiple ARGB images	1	38	0	0	0
Rotated ARGB images	29	34	9	9	0
Multiple labels	3	40 (+7)	0	0	0
Screen sized text	81 (-1)	45 (-1)	17 (-1)	17 (-1)	0
Multiple arcs	33 (+6)	33	6 (-1)	6 (-1)	0
Containers	5 (+5)	37	0	0	0
Containers with overlay	87 (-12)	23	42 (+1)	42 (+1)	0
Containers with opa	16	38	0 (-1)	0 (-1)	0
Containers with opa_layer	8 (+1)	37	1	1	0
Containers with scrolling	45	46	12 (+1)	12 (+1)	0
Widgets demo	66 (+1)	42	15	15	0
All scenes avg.	24	38	6	6	0

Disclaimer: These benchmarks were run in an emulated environment using QEMU with instruction counting mode.
The timing values represent relative performance metrics within this specific virtualized setup and should
not be interpreted as absolute real-world performance measurements. Values are deterministic and useful for
comparing different LVGL features and configurations, but may not correlate directly with performance on
physical hardware. The measurements are intended for comparative analysis only.

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[cubic-dev-ai]

1 issue found across 16 files

Prompt for AI agents (all 1 issues)

Understand the root cause of the following 1 issues and fix them.


<file name="src/layouts/flex/lv_flex.c">

<violation number="1" location="src/layouts/flex/lv_flex.c:373">
Multiplying grow_max_size by grow_value before dividing can overflow int32_t (e.g., 9,000,000 * 255 &gt; INT32_MAX), leading to undefined behavior and wrong flex sizes when large coordinates are used.</violation>
</file>

_{React with 👍 or 👎 to teach cubic. Mention @cubic-dev-ai to give feedback, ask questions, or re-run the review.}

[AndreCostaaa]

Makes sense to me. Thanks!

[Copilot]

Pull Request Overview

This PR fixes a rounding issue in flex layout where grow_unit was always rounded down, sometimes leaving empty space on flex tracks. The fix uses rounding to nearest integer instead of truncation, similar to the existing grid FR implementation (#6255).

Key Changes

• Added div_round_closest helper function for rounding division
• Changed flex grow size calculation from truncating division to nearest-integer rounding
• Added test case to verify flex tracks are completely filled with growing items

Reviewed Changes

Copilot reviewed 2 out of 16 changed files in this pull request and generated no comments.

File	Description
src/layouts/flex/lv_flex.c	Implements rounding fix by replacing truncating division with div_round_closest
tests/src/test_cases/test_flex_grow.c	Adds test case for flex grow behavior with various combinations of grow values and fixed sizes
tests/ref_imgs/*.png	Updated reference images reflecting the fix

---

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[W-Mai]

This looks good, but I'm wondering if simply adding rounding operations can really avoid all errors? For example, the current grow is 1:1:1, but the total width is 10. Can the current algorithm solve this?
I'm curious about how it works. Thank you.🚀

[niklasf]

The 10 into 1:1:1 example was working even before. That's because each iteration of the loop only looks at the remaining grow_max_size and grow_value_sum.

grow_max_size	grow_value	grow_value_sum	grow_unit	size
10	1	3	10 / 3 = 3	1 * 3 = 3
10 - 3 = 7	1	3 - 1 = 2	7 / 2 = 3	1 * 3 = 3
7 - 3 = 4	1	2 - 1 = 1	4 / 1 = 4	1 * 4 = 4

But lets look at 10 into 4:4:4 for an extreme example, which should be similar. Before:

grow_max_size	grow_value	grow_value_sum	grow_unit	size
10	4	12	10 / 12 = 0	4 * 0 = 0
10 - 0 = 10	4	12 - 4 = 8	10 / 8 = 1	4 * 1 = 4
10 - 4 = 6	4	8 - 4 = 4	6 / 4 = 1	4 * 1 = 4

Instead, with the improved rounding, each element is at most 1 off the ideal size, and the last iteration will have no rounding at all:

grow_max_size	grow_value	grow_value_sum	size
10	4	12	div_round_closest(10 * 4, 12) = 3
10 - 3 = 7	4	12 - 4 = 8	div_round_closest(7 * 4, 8) = 4
7 - 4 = 3	4	8 - 4 = 4	div_round_closest(3 * 4, 4) = 3

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Rebased to fix conflict in reference images.