bug: InteractiveViewer cannot shrink to the specified scale

[pychans]

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• I have searched the opened issues and there are no duplicates

Describe the bug

In the latest version of official code(0.70.0.dev6370), InteractiveViewer cannot be scaled down to 0.1 min_scale, but can only be scaled down to the minimum of 0.8 by default

Code sample

Code

import flet as ft


def main(page: ft.Page):
    page.add(
        ft.InteractiveViewer(
            min_scale=0.1,
            max_scale=15,
            # boundary_margin=ft.margin.all(20),
            # on_interaction_start=lambda e: print(e),
            # on_interaction_end=lambda e: print(e),
            # on_interaction_update=lambda e: print(e),
            content=ft.Image(
                src="https://picsum.photos/500/500",
            ),
        )
    )


ft.run(main)

To reproduce

Steps to reproduce:1. Run the repro code 2.Scroll the mouse wheel to zoom out the picture 3.It will be found that only 0.8 min_scale can be reached

Expected behavior

No response

Screenshots / Videos

Captures

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Operating System

Windows

Operating system details

Windows 10

Flet version

0.70.0.dev6370

Regression

I'm not sure / I don't know

Suggestions

No response

Logs

Logs

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Additional details

No response

[ndonkoHenri]

I opened a PR to make the documentation a little clearer:

min_scale: Number = 0.8
"""
The minimum allowed scale.

The effective scale is limited by the value of [`boundary_margin`][(c).].
If scaling would cause the content to be displayed outside the defined boundary,
it is prevented. By default, `boundary_margin` is set to `Margin.all(0)`,
so scaling below `1.0` is typically not possible unless you increase the
`boundary_margin` value.

Raises:
    ValueError: If it is not greater than `0` or less than [`max_scale`][(c).].
"""

Basically, you need to increase the boundary_margin, for example to ft.Margin.all(20).

Let know if you face additional issues.

[pychans]

It may be that I didn't consider it fully enough, but I intuitively felt that the design was a bit counter common sense. Normally, an interactive target needs to be zoomed in and out by default only by considering the scale, rather than the conventional zooming out, which requires the configuration of boundary parameters.

[ndonkoHenri]

It may be that I didn't consider it fully enough, but I intuitively felt that the design was a bit counter common sense. Normally, an interactive target needs to be zoomed in and out by default only by considering the scale, rather than the conventional zooming out, which requires the configuration of boundary parameters.

I dug into it a little: Flutter’s InteractiveViewer clamps the transform whenever the viewport would escape the boundary rect. To keep the requested scale, we'll have to predict and resize that boundary in sync with layout (including async content, transforms, constrained vs unconstrained parents, etc.).
Every prototype I tried ran into those edge cases, so for now the pragmatic fix is to set boundary_margin yourself, as discussed above.

If someone has a better idea/approach, I’ll happily revisit.