Correct events to restore width/height with non-default DPI
lazarus / 21-General

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anse · 2025-03-27 08:51

I'm still having some confusion with restoring the width and height of
a form, especially when the form has DesignTimePPI=120 (not 96), and
the app runs on a different DPI setting. Currently I am storing a
user-defined size of a form in the OnDestroy event of a form:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TMyForm.FormDestroy(Sender: Tobject);begin 
SaveToJson('TMyForm.Width', ScaleFormToDesign(Width)); 
SaveToJson('TMyForm.Height', ScaleFormToDesign(Height));end; procedure
TMyForm.FormCreate(Sender: TObject);begin  // Restoring size  Width :=
ReadFromJson('TMyForm.Width');  Height :=
ReadFromJson('TMyForm.Height');end; 
Is that the right approach when I want to support DPI scaling across
platforms, plus consider a value in TMyForm.DesignTimePPI <> 96?
Does it make a difference whether I store the width/height on
OnDestroy or OnClose - or in other words - is a form "DeScaled" in
OnDestroy? I guess no.

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anse · 2025-03-29 16:03

To answer my own question: yes, that approch is working correctly.

Here's a screen of a test project
* designed on Windows @120ppi
* then copied to my Linux box
* built with /usr/bin/lazbuild (without touching the form properties)
* then running on Linux @96ppi.

Observations:
* DesignTimePPI is 120, and never changes at runtime
* PixelsPerInch was 120 at designtime, changes at runtime to the value
of Screen.PixelsPerInch
* FormCreate sets width and height to 500px (still in 120ppi as
designed!)
* LCL scaling takes place (where exactly?)
* in FormShow, the width is now 400px (96ppi)
* In FormDestroy the form is not descaled as I suspected
* Instead, storing the width for the next session using
ScaleFormToDesign(Width) scales it up from 400px to 500px
* Next time the app runs it sets the width again to 500px, and the
scaling again decreases that to 400px

Just feels a bit wrong to store dimensions in the scaling the
developer dictates, instead of the actual user scaling. But that's
rather philosophic I suppose.