fix: panel flex-grow may less than 1 after multiple layouts

[aircloud]

try to fix an issue:

When I use docMove to add a panel below some panel-A, and then delete it, the panel-A size will be directly halved, After a few times like this, the size of panel-A is less than 1.

According to the calculation rules of the panel, when the flex is less than 1, it actually cannot fill the entire area This causes panel-A to be smaller and smaller.

rule:

// when less than 1, it will not fill the whole area
let flexGrow = flex * size;

I don't think the halving is necessary here, so I'm trying to submit this PR, and if the author thinks there are other issues with this, please tell me.

Thank you!